I suppose it depends on whether your application depends on ephemeral disk random or sequential I/O, EBS I/O, I/O to the internet at large, cpu cache, ram bandwidth, support for AVX instructions and so on.

To be fair it's understandable why Amazon doesn't promise these features will or won't be present - it would make their already-complicated product offering even more complicated. And for a great many applications, customers won't be sensitive to details like CPU cache and disk performance.