I think you painting quite a dark light on the old a500 which happned to be the most successful amiga. As far as I know ECS only programs are few and far between and you did get the ECS chipset in the a500 during the end of the KS1.3 era (which happens to be the a500) i own, also with it been the most successful model it also makes it the most readily available meening you can pick one up for bugger all if you shop arround i got mine for £5 .
As far as the a600 goes and i know this opinion probably wont be popular i have always considered in the jack of all traids but master of none machine. When it was launched i did think whats the point when the a1200 was only a bit more. Now I am older and wiser(?) I can see its place, and will probably get hold of one.
As far at the a1200 incompatabilities these have plagued amigas since the started upgrading the kickstarts probably the biggest problem was when they jumped from ks1.3 to ks2.0 . Part of the reason i suggested a a500 ks1.3 for the best compatability .