6 shots isn't a huge problem in and by itself -- I realcap load my 1911 mags (8 rounds) without issue.
However, a revolver forces you to either shoot Double Action (a long, hard pull -- at least on my revolver) which adversely affects accuracy or manually cock each shot, making follow-up shots extremely slow.
Reloading a revolver is also very slow, even with a speedloader, and made worse in airsoft because we have to save our "spent" brass. I know there's some revolver shooters that can reload extremely fast, but unless you're a cowboy action shooter that probably isn't the case.
Point in case, I can fire off three 1911 mags (24 shots) in almost the same time it takes to fire 6 shots and reload a revolver.