No explanation is particularly necessary, given that Namor is not in the series at all. And I think the actual explanation is right there in the pages of AVX. Namor was the first to lose the power of the Phoenix, and him doing so meant that the remaining four Phoenixes were jolted with a sudden influx of greater power. This was the point at which they all really started to go a bit mad, especially those that held onto the power the longest, and who absorbed similar jolts when the others relinquished it. I think the dots are pretty easy to connect here, and that it was this escalation of power, a power they should not have had and an influx of power that they were not prepared for, that screwed up the genetics of the other four mutants, making their original powers unstable.
Sometimes I think people just complain to complain; especially when it’s about a popular writer. Some people just have to be contrarians, I guess.