I purchased an HK91 at a time when they were cheaper than most of the other rifles on the market. I compared them to what was used by the previous generation of soldiers, the M1A, Mausers, etc.
In terms of recoil, the two part cycling of unlocking the bolt and then rebounding it off the buffer meant it was a lower impulse that was spread out longer. Much less perceived force compared to a bolt action, it was a softer shooter, and easier to control plinking off full mags. We did that in the day, surplus .308 was 25c a round.
The ergonomics were fine - you hold a gun up at the shoulder and off hand, it had some drop in the stock with the sights a bit elevated using the claw mount. It was plenty accurate enough, the barrels were hammer forged even then. Most Euro battle rifles did that because their investment in the future was assured by government contracts that embraced improvements, unlike Colts arrangement with the US Army. Compared to the existing guns - they were all a bit different, and they all had drop in the stock in the traditional manner. It's the straight line buffer design that forces more work arounds in some respects.
Accessories, not so much. Like any other non-AR design, it's single source and proprietary. Back then the dollar vs mark exchange was decent, but only "GI" parts existed. The private market couldn't create enough demand for newer accessories for a gun that could be seen to be in it's latter stages of fielding. All the contracts were mostly finished for first tier nations, and the second tier buying them took what they got with few modification. So all we had were slim line Euro or fat tropical handguards. It wasn't until two decades later, after I had sold it, that magazines dropped under $10, wood or OD furniture became available, etc. In terms of accessories, things are better now than ever. Then we blocked a standard 20 round mag to ten for hunting purposes as the five rounders were $45. Couldn't afford it.
In use, they were no heavier than other .308's of the day, and there were plenty around in the early days of 3 Gun against the FN and others. But the design limited being able to tinker with it, so it was left behind in those circles. That is it's biggest limitation - there is very little anyone wants to do to alter timing, as it's mostly controlled by the cam shaping on the roller lock nose of the bolt. Nobody wants to chance grinding them wrong or experimenting. There's very little public knowledge and it's high level gun designer work.
Overall, they are good rifles, you take them as they are in the most part. What has undercut them and the other .308's is the huge availability of the AR and it's modular design that allows the owner to change it at will. A more important aspect is that the .308 has three times the recoil impulse - and it two pounds heavier. The average soldier can shoot the intermediates more accurately and more often. A range trip with a .308 is usually limited to less than 100 rounds, the AR often double or triple that. It's exactly why the military went to smaller calibers - it's easier to shoot, so more shots are taken. More bullets in the air means more hits and the enemy suffers more combat power loss.
I pretty much shot my HK91 as is, for what it was. My AR, nope, I built it originally as a 6.8 dissipator with 16" barrel and rifle handguards, A1 stock. It's now being transformed to an adjustable carbine buffer with MFT Minimalist and Apex free float. That sort of thing is nearly impossible with an HK, and the AR is much easier and more pleasant to shoot, even with the bigger bullet of 6.8 - up to double the grain weight. I don't miss the HK much at all, and the deer don't seem to notice any difference either. They are still shot and fall down from over 1,000 foot pounds out to the visible range I can see them.
