I've had a Lee ABLP for about 5 years with all the accessories that make it a "4000." The 4000 didn't come out as a package until later. I moved away from it a little over two years ago but I still use it for a few operations.
Progressives are no good for producing precision cartridges from start-to-finish. I know some people will want to argue with that right off the bat, but how many of them are trickling on their progressive? Another major shortcoming of loading from start-to-finish with a progressive is that reloading is really divided into three major steps: brass cleaning, brass preparation, and loading.
Brass cleaning can't be done on the press. High-volume, low-precision shooters won't care about primer pockets or maybe even cleanliness altogether, but I don't have any "plinkers." Many progressive reloaders will load dirty brass on their presses, but some could decap and clean their brass before bringing it to the press. Even if the brass is cleaned before decapping, it still cannot be annealed on the press.
To resize, carbide dies or lube is used. Carbide dies rarely offer the same case to chamber conformity as steel dies, and of course they're not generally available for bottleneck cartridges. If the cases are lubed before sizing, they'll have to come off the press again to be cleaned before they're loaded. There are some polymer-based lubes like One-Shot that can be left on, but I've found that those lubes work well with carbide dies and straight-wall cartridges but not so well with harder resizing of bottleneck cartridges in steel dies. Once the brass is sized, it will likely need to come off the press for trimming. There are some on-press trimming methods, but the best methods to get square case mouths are off-press.
To load the brass, most progressive users employ a case-activated powder measure. These meter by volume which has limited precision, especially with stick and flake powders. The only way to get precision is by trickling. Because they often depend on the case mouth to impart the force to activate the linkage for the powder drum or disc, the case mouth can easily be damaged. Precision reloaders put a lot of effort into case mouths and necks, and then the progressive reloading smashes it against the powder measure. No thanks.
The progressive press does have some features that are of high value. The case feeder is a big time saver. Because the cases must come off the press frequently as explained above, a case feeder is a tremendous help. Even after I shifted away from loading on a progressive, I continue to use the progressive with the case feeder to decap. It's a great decapping machine. The case feeder also makes loading brass for resizing straight-wall cartridges more efficient. I won't resize bottle-neck brass on my progressive because I find the Lee ABLP way too flimsy to achieve precision in shoulder bump. I believe it could be done with a heavy-duty Dillon or similar. For handgun brass, I am using three dies to size and expand the cases. The progressive with the case feeder works well for this operation. I will say that the Lee case feeder works well enough to be helpful, but otherwise it is troublesome. I've learned to minimize jams and malfunctions and also to clear them quickly, but it is far from being trouble-free. It is cheap though.
Once I've loaded the cases with powder, the case feeder is not useful, so I would not go back to the progressive for anything beyond that. When I did load on a progressive, I used a drum powder measure and an RCBS lockout die to check powder levels. I also used a Lee bullet feeder, but found it worked poorly and changes from one bullet to another were finicky. Now I visually inspect all the powder charges in a loading block before placing bullets.
Seating bullets is another precision operation that I think is best performed on a single stage. I had a LCT with the case ejector. Once a bullet is seated, it could be crimped and ejected in a turret or progressive. There's some efficiency to be gained there. Because I want the precision of a single stage for seating, I learned to hold two cartridges with the fingers of one hand. I can pull a cartridge out of the shell holder and place the next one in it without having to go back and forth from the loading block. My hand moves to place and pick the next cartridge from the block while the other arm is pulling the lever. I keep the block close so less movement is needed.
. Currently using a Lee turret press and I sometimes do steps individually for more exactness. Anyway, the main hesitation is potential frustration level of dealing with finicky tools when exacting precision is necessary. Reading various thread here has taught me that no one brand is going to eliminate problems and that no matter what I go with, I will have to expect problems which I will need to take time to overcome. Plus money. Funds are not unlimited and I am just not spending thousands on a machine. So, I have been leaning toward the Lee 6000 since the price is right and it seems people are happy with it. Plus I already have experience with Lee and a couple of potential issues (I can probably take their Auto Drum Powder Measure completely apart and reassemble with my eyes closed 
