The WE open bolts, or KJW M4's are highly reliable
Unfortunately most of the opinion on GBBR's come from WA's abortion of a design. If you buy a WA, expect to spend $400 just to get it working, and expect it to break regularly.
With a KJ or WE open bolt, it's not uncommon to go one or two seasons without the gun experiencing any problems. The mags are usually the focus of problems on most GBBR platforms, but it's typically simple gasket leaks that can be easily fixed with gasket sealer.
However, the 'problem' with GBBRs is not the reliability, so much as the performance.
They work amazing in 18-24 degree weather, but if it gets too hot, they shoot too hard and are very prone to breaking. If it's too cold, you'll only get a few shots out before they stop. Also, due to the way gas guns work, you'll shoot harder with heavier ammo, so you need to chrono with the rounds you'll be using, not with .20g BBs. And related to that, as the temperature rises outside, so will your FPS, it's possible for a GBBR to jump 50-100fps from morning to noon. And you're typically limited to 30 (very accurate and long range) rounds per mag.
By comparison, an AEG can work in temperatures of -5 to +40, NiMH batteries die faster in the cold, but LiPo's do not. Parts are fairly cheap and always readily available. There's less maintenance involved. And it's very possible to have a stock gun last 2-3 seasons before breaking.
I know lots of guys that bought GBBRs and ended up switching to AEG's because they could only afford one gun. They're cool, but they're not work horses