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The blitter problems is to do with lazy and 'clever' programming.
The Amiga was a proper multi processing computer, so whilst the blitter was doing its thing, you could have the CPU doing something else.
To help speed up programs, programmers would use the blitter to do something, and then not wait for it to finish and let the CPU continue through the code. Because the blitter was significantly quicker than the 68000 processor, invariably the blitter would be finished doing its job before the CPU gave it its next job, therefore there was no need for a piece of code to wait for the blitter to finish.
Obviously the 68020 in the A1200 is quicker than the 68000, so the same code for the blitter was failing because the 68020 could process the code quicker and was already setting up the blitter to do another operation before the blitter had finished its previous job, hence the garbled graphics.
And obviously the faster 68030/040/060 just make it even worse.
Whats makes the blitter even more of a headache is that the 68030+ are quicker than the blitter at doing the same job, you really have to question the intelligence of Commodore to maintain a custom chipset that then ceases to be amazing and ends up being a bottleneck, strangling the performance of the AGA Amigas.
Its not enough to say its for backwards compatibility, because Amiga programmers relied on those custom chips to do the hard work instead of the processor.
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