As an owner of a WE AK74UN I'll chime in with my opinion. Here's mine after owning it for 2 years now(And accidentally making it look like the Chinese Assault Rifle from Fallout 3 for a Fallout themed event):
First, I'll point out the negative parts, mainly parts that broke down.
-When I first got it, I ripped apart the hop up bucking because of a cracked bb in the chamber. On the plus side, because of the sheer weight of the bolt, any bbs jammed in the chamber will get crushed and continue working like it was nothing
-I replaced the trigger unit with the RA Tech version after 2000 rounds because the hammer snapped. The trigger still works if I JB Weld the hammer back in place.
-Broke the nozzle in 2 during a CQB game, no biggie was bound to happen eventually.
-The slow rate of fire from the full travel bolt not only affects it in full auto, but also in semi because of the travel time.
-The plastic 74 magazine shells are easy to break, mainly at the bottom where the shell is screwed to the internals.
Aside from that, here are some of the positives about the WE AK74UN:
-The blowback feels really strong because of the weight of the bolt.
-Magazines are fairly cheaper than the others and doesn't have any leaking issues like the other WE GBBs
-Because it's WE, replacement parts are easily available, and there's always aftermarket parts for it.
-It's pretty durable like the real steel version as in it will work no matter what you do to it, which probably doesn't mean much compared to the other brands because of what it suppose to replicate.
-Takes the same inner barrel and hop up bucking from a Marui GBB and WE Open bolt, so upgrading that should be easy. Speaking of open bolt, it also accepts the RA Tech WE open bolt NPAS.
-Because of how cheap it is compared to other GBB AKs, you can afford to give it the weathered look naturally.
-Very little maintenance needed. I use Froglube on all the metal parts, and silicone lube from a sex shop to maintain the rubber parts.
-Hop up is easy to adjust
-The gun stops firing when there's no bbs in the magazine via a small switch in the magazine lifting the part that hits the valve that releases gas up. What that means is that unlike other GBB AKs out there, the bolt will not lock when the magazine is empty like the real steel version. For a while, only WE was able to do that until GHK did it after the WE AKs were released.
-And lastly, only WE AKs have full bolt travel atm so if you want a GBB AK that you can pull the bolt all the way back, go WE.
I haven't tried the other GBBR AKs, so I really can't compare it with the others. So hopefully this helps.