Poor accuracy, poor metallurgy, poor stocks, poor function, poor reliability for the autos. Poor sights. Poor triggers. Other than the fact that they are physically capable of chambering a round and firing it, it's hard to find any good points on any of the three.
Warning--wall o'text post ahead. And this is my personal opinion and as usual YMMV.
Actually, from the Russian arms that I have seen and dealt with, including rebuilding from parts on some, the metallurgy is fine, leaving aside times like the Russian Revolution or perhaps the fall of Soviet Union. Metallurgy is one area that the Russians have done pretty well and actually I've seen less corrosion problems on Mosin parts such as bolts and receivers than Mausers, including the vaunted German ones. Russia also chrome lined bores before it became fashionable in the West. German metallurgy during WWII and somewhat during later stages of WWI also demonstrated problems. The U.S. had the obvious problem during WWI with the Springfield 1903's manufacture.
On particulars, the SKS for example, works fine, is rugged, and about as accurate as its competitors as issued including the Garand. The issued trigger is not great but is similar in design to the Garand and can be tuned to be quite acceptable for a semi-auto. No credible evidence exists that that Russian semi-auto/automatics are unreliable--there is a reason that the Israelis chose to model their Galil on the AK and that the Finns, despite being bitter about the Russo-Finnish wars, adopted an AK variant--the Valmet for a time. They are more reliable than most designs if abused. Personally, I have never had a stoppage nor feed issue on my SKS despite firing some rather cheap steel cased ammo. I have also got around 2 MOA from that rifle using handloads and Tech Sights.
I do not have an AK but respect its reliability and ruggedness. I don't have one because I have other rifles that are as reliable and rugged with better sights, better triggers, and accuracy. If you get some of the high end AK's or AK variants, I understand that these have better triggers and accuracy with provisions for sights. The AK-74 apparently is more accurate in general but I never acquired one because the ammo availability was questionable. A buddy of mine has one and likes it.
On the use by Russians of open sights, I somewhat agree. But, the open sights on these firearms come from a different military doctrine than the U.S. where volume of fire and peripheral visibility were more important to Russians than every man is a rifleman ideal of the Americans. Russian sights on the Mosin are not that inferior to similar Mauser open sights and the greater 5+ inches of barrel gives a longer sight line. Ceterus paribus, longer sight lines improve open sight performance. Mosin sights were also hooded and protected as issued compared with Mausers. Firing them, I do not have a problem despite being trained on peep sight types. SKS open sights are similar but obviously on a much shorter rifle and firing a cartridge with much shorter effective range AK's sights, I do not like particularly but they are no worse than many sub-machine gun sights. Russian doctrine was to fire the Mosin with the bayonet attached from what I recall. Some of the reports of inaccuracy stem from firing the rifle without the bayonet which obviously gives a different POA. The Russian sniper rifles using the Mosin platform and the Finns wringing out every bit of goodness out the Mosin indicate that the platform itself can be as accurate as any of their competitors in bolt actions.
The AK was viewed as a bullet sprayer replacing their pistol caliber sub machine guns from what I understand about Soviet tactics. Volume of fire was more important than minute of angle accuracy to the Russians based on their WWII tactics--the 5-6 MOA accuracy was not problematic if you are firing full auto and relying on volume of fire/human wave tactics. They had special sniper rifles and snipers trained for aimed fire and used these far more than the Americans and British during WWII (Germans also had snipers but do not believe it was integrated as much in their army than the Russians from what I recall).
U.S. rifles of the time had more sophisticated and slower to deploy sights. Probably because of logistics of transporting ammo across the US, we did not emphasize mass firing in our military training. Note that the U.S. did not use volley type sights between the Civil War and WWI and had an almost pathological fear of troops wasting small arms ammunition using magazine cutoffs etc. The 1888 GEW and subsequent Mausers never had magazine cutoffs nor did the Mosins. Thus, we had to focus on marksmanship and making each shot count. The British were in-between. They retained the volley sights far later than they were useful as a remnant of the musket era but scrapped them in WWI. However, they developed some of the best adjustable open sights on the original No. 1, went to peep sights in the P14, and then adopted them wholesale for the No. 4 rifle.
Triggers for late 19th and 20th century U.S. and British bolt actions are generally quite good unless Bubba has been ahold of them. Mausers vary from decent to awful without tuning as they were designed for functioning under rough conditions and ease of manufacture. Steyr Mannlichers, French bolt actions, etc. vary in their suckitude but also go bang. Apart from the weird MAS 36 type sights, these also use open sights of varying quality. Probably the best issued triggers were those of the Swiss but even they kept open sights for general issue weapons.
On U.S. issued weapons, triggers for U.S. semi-autos vary in quality with the AR series issue triggers not being very good compared to the U.S. bolt rifles or even the Garand/M14/M1 carbine. There is a reason that many seek aftermarket triggers for AR's.
Never fired a FAL nor British bullpup so others will be better judges of those.
Regarding stocks, the Mosin stocks are not much different than U.S. 1903 S stocks. These were never designed for scopes. C stocks were an upgrade for U.S. forces using the 1903 with scant stocks in the middle. Two piece stocks (buttstock and forend) can be inferior in accuracy and do require more careful fitting than a Mosin or U.S. stock but use wood more efficiently. The British did quite well with theirs because of their traditions of armorers. AK and SKS stocks are similar two piece stocks and both bear a resemblance to MAS/Lebel type stocks. The SKS laminated stock is quite nice but heavy and the AK has went to synthetic. Both usually use native available woods rather than walnut. But birch, apart from aesthetics, is a decent wood to work with and is comparable with walnut. Both are designed for open sights and neither for scopes. However, compared with the AR's carry handle/sight combo, neither was the original AR's stock. Putting a scope on an AR with the integral carrying handle is more awkward than the SKS or AK.
I don't find a Mosin perfect--the obvious issue is rimlock and the necessity of having a cartridge interruptor due to using a rimmed cartridge. The second is the safety is poorly designed--I suspect in practice they did as the French (the Lebel, Berthier, and MAS 36 have no safeties) of loading when ready to fire and otherwise keeping the chamber empty. The trigger is not very good as issued and quality of manufacturing dropped at times during the Russian Revolution or the Great Patriotic War. Aesthetically, the Mosin's bolt handle is short, stubby, and looks awkward compared to the German Mauser. Fit and finish of the rifles is not up to par with pre-war German, French, or English rifles. Shellac is not that durable for a rifle finish and the chipped, gobbed on, poorly stained stocks detract from its somewhat suspect beauty and if stripped, birch stocks rarely show great figure. That being said, it is a historic rifle being used in multiple wars, has a clever and unique design, available and relatively cheap, has cheap ammo available to shoot it compared with some milsurps, and it pretty much goes bang when you pull the trigger regardless of conditions. It is collectible for all of those reasons. The Springfield, Mauser, Enfield, and Mosin, did things that changed the world for better or worse. It is natural that some people want to acquire them and some to shoot them.
Some of those things that were done with those rifles, created death, misery, and destruction. I can understand people not wanting certain rifles for those reasons. People also like to argue the merits and demerits of particular rifles including design, function, accuracy, battle doctrines, etc. As for myself, I like these rifles' connection to history and am willing to forgive their particular shortcomings just as I revere passed down family firearms for the same reason. 'Nuff said.
