"Rendering programs, anything using a lot of ram image editors etc, sound editing progs etc. Anything that take things and applies large batch style processes to them etc."
How many of these depend on the native chipset? Even the oldest version photogenics I remember tinkering with would run on RTG as well as its HAM modes.
"Games etc it won't matter to in most cases, with faster new hardware you'd possibly see less chugging than on a standard amiga chipset, but you also might get the game running at many multiples of the original speed if it hadn't been coded to avoid that. "
UAE has a speed control though doesnt it? Ive seen an option to match A500 speed. Im sure it wouldnt be impossible to do the same for A1200 and some other base models.
"Then there's compatibility issues with emulated hardware. UAE is pretty good at dealing with most of them but there's still a few."
Perhaps but then any new hardware emulation would have some problems too - especially when you have to graft it into a quite alien system. At least we know what UAEs problems are and its also lot easier to fix software than hardware.