Shimming and other AEG mechanics aren't a problem in and by themselves when you know how to do it (albeit tedious on lower quality mechboxes), but a major downside is the difficulty/effort of getting to the mechbox in the first place: if its relatively simple on some guns, other require disassembling the entire gun just to get to the mechbox. On M4s and other V2 gearboxes, you can't test the mechbox without reassembling the whole gun (or a lot of it) either. Just to disassemble it again if you need to change something.
GBBRs are mechanically simple: there's only a small handful of moving parts. For my WE M4s, for example, I carry an extra spare bolt carrier group and a spare complete trigger box. They can be changed in a matter of minutes (seconds, for the spare bolt carrier) and that will fix most problems that could occur on the field. The only other thing would be a leaky mag (you're down one mag), otherwise someone needs to run over your gun with a car or something.
GBBRs will operate reliably in wet and extreme wet conditions: in fact you could leave them underwater for a while and they'd work fine. Obviously you don't want to fire an AEG while its waterlogged, you don't want to get a MOSFET or other controller circuits wet (e.g., PTWs etc) and you don't want to risk shorting a battery, specially a LiPO.
You also don't need to deal with wiring and battery-carrying issues. If you do a lot of custom work, specially if you want to create a small compact package, this is an important consideration. Wiring always tends to be problematic, too: how many times have I see, on game mornings, people with battery and/or wiring gun problems? Its happened to me on a few occasions, too. Wires get accidentally yanked, they break their solders, connector snap, wires get pinched, etc.
With an NPAS you can easily adjust your FPS, removing the necessity of having a separate "indoor" and "outdoor" gun. Yes, there are some mechboxes with quick swap springs, etc. but its not all AEGs, and it'll often force you to go into a certain platform (PTW, WE's Katana, etc) or buy an expensive mechbox. NPAS for WE is $20.
But for me, really, its a combination of all those little factors:
Someone could call me right this second and tell me we're going to play, Ijust have to grab my gear, mags, gun, BB and gas and I'm out the door. Compare this to years of Night-Before-Gameday spent charging a battery, a spare battery, test firing the gun to make sure the batteries are okay, checking the FPS to make sure the gun is okay, packing a spare gun (cuz if something happens to your main AEG, you may not be able to fix it on the field: occurrences of that happening are legendary), charging batteries for the spare gun, testing the spare gun. It's like 2AM by now, I have to get up at 5 and drive 2 hrs to get to the field, FFFFFUUUUUUUUUUU.
Properly tuned, GBB FPS won't always fluctuate that much. I find the accuracy quite good, better than a lot of AEGs I've used (AEG FPS can fluctuate too). They accept a lot of RS parts and the balance more realistic (ie., no heavy battery on one end). The trigger response is obviously lightning fast and you can double tap like the real thing. A proficient shoot will experience a clear edge over most AEGs: I can get two shots off in the time it takes a lot of AEGs to get their first shot fired. Only highly tuned guns, PTWs etc can compete at the same level.
And obviously it's fun as fuck to shoot. Completely new experience that breathed a second life into airsoft for me.
Last edited by Drake; March 24th, 2013 at 12:47..