I've been using Andrea and Vallejo acrylics for several years now on (mainly) 54mm medieval foot figures. Recently, I began using Reaper acrylics and Liquitex Slo-Dri and Fluidant (a flow extender). I've also been paying more attention to layering and glazing to get better transitions. As for larger scales, I have used them on 75-80mm medieval foot figures as well and I've had no difficulty; I see no reason why they can't be used on even larger scales like 120mm or 200mm busts.
Like GW, Reaper paints were designed for use on gaming miniatures - generally 30mm and smaller. While I've found all of the paints are chemically compatible, I wonder if Reaper (and other gaming associated paints like GW) might have a finer ground pigment. They do seem more transparent, but that's easily solved with a tweak in painting technique.
I can't imagine GW paints aren't as equally good, especially after spending time looking at gaming minis painted as stand alone art pieces. Wow...
Cheers,
Glen