All retail outfits will have horror stories of some sorts but some are far better than the average. If I'm going to spend my money it will be with the shop that's far better than average. If someone else wants to use a half decent tech then by all means...
Upgrading an AEG, regardless of brand, isn't rocket science I agree but experience plays a big role in the outcome. KOA has tons more experience than I do and I know quite a few people who have used them just as much as I know quite a few people who have used other shops. IMO KOA is the place to go for TM upgrades as they know the brand extremely well, have a very good reputation with TM upgrades and have strong ties with TM part suppliers like Eagle6 which makes the process a lot smoother.
If you are going to modify your BMW sports car would you prefer any half decent race shop or would you prefer one that specialises in BMWs and has a long track record of successfuly modifying BMWs?