Another GHK G5 lover here.
You have to go in to GBBRs eyes open. Your mags have far less amo (GHKs are ~36 for M4 mags, 45 (?) for AK) so if you play at a spam fest you may well feel under-gunned. Also G5 is affected by the cold, just not necessarily as bad as others. In ~2-3 degrees C I was managing a whole mag, but the last few shots were starting to show signs of cool down (not from rapid fire). Also mags are a LOT more expensive and you need more of them. I carry 6 - which weigh a lot and in this country would cost you ~£250 (plus the one that came with the gun).
As to reliability - they are just as reliable as AEGs however need more (simple) maintenance. You just need to clean and lube them after every day you've been playing, plus leave gas in the mags. Follow the basics and they won't go wrong on you.
Oh one other downside - they eat optics. The cheap airsoft optics can rarely take the recoil of a GBBR (and some of the more aggressive recoil AEGs). After two optics (and a torch) shaking apart on me I've gone real steel (at the lower end - a Vector Optics unit).
Half tempted by a GHK AK myself, but can't justify it.