The Amigas "on paper" specs are quite different to what can be realised. For instance the 16 color per playfield (or 8 for ocs/ecs) limit is technically per scanline and palette registers can be changed multiple times. Hardware sprites are also more flexible than most people give them credit for. Even on ocs/ecs Ive had dozens of 16 color hardware sprites happy moving around (and this of course doubles for 2bit color sprites). AGA sprites are even more powerful (although have some restrictions that ocs/ecs dont too unfortunately, like it being nigh on impossible to have more than 1 16 colors 64 pixel wide sprite on a 4+ bit color screen without narrowing the screen (can multiplex that sprite though).
Anyway the point to all this I guess is a set of universal tools for accessing the amigas custom hardware is never going to get the most from the system. Use one trick and other things are closed off while other things are opened up, and so on and so forth.
In part I guess this is why demo scene coders still use amigas, there's no real definite restrictions/limits, it's almost an artform unto itself.
p.s. Speaking of transparency, one of my efforts to do a streetfighter2 remake for amigaos ended up with one of the characters ended up transparent. Was actually a bug rather than intentional, but was "free" transparency nonetheless. One of the most interesting bits of code I did for ocs, and it was accidental