Generally the more complex the better for my money, although there is a line.

ROTF Leader Prime and Mixmaster were for me on the right side of that line. All three MP Starscreams were as well (original MP, 2012 update, and the movie masterpiece).

ROTF Skids was not. MP Hot Rod was also not the right side of that - it wasn't massively complex but it was too fiddly, and not particularly intuitive. I've heard a lot of negativity along similar lines about the Alt Grimlock/Wheeljack mold.