in gearbox configuration you're looking at manufacturing tolerance... in a worst case scenario if you needed to replace the whole gearbox shell, will any drop in fit? There's lots of stories of shit being off by fractions of a mm and making things really hard to fit together properly among other brands. Those are the absolute worst horror stories, spending weeks/months trying to track down why you're losing air or not feeding.
To a lesser extent, things like standard parts for v2/3 gearboxes fitting or not fitting. Some gearbox shells don't fit certain cylinder heads for example, or some reinforcement bit being too thick or thin for X part. In most cases these are things you can work around once you find out about them.
Nozzle length is usually the kicker, does it have a proprietary length or can you just drop any aftermarket ak or m4 or whatever nozzle in? does the stock nozzle have an o ring? If you can't replace it/upgrade it down the line it will be a pain in the ass to keep it running.
I futz with V2s, V3s and V6s. I could be wrong... but probably, most likely not, as far as I know.