Kai is largely responsible for Angra's career, compositional development, production and sound. He has guest solos AND vocals all over their catalog, and much of their material is produced at Hansen Studios.
Edu is an amazing singer - Kai is at least an equally amazing singer (preference on that is just a matter of opinion). However, saying 'Kai cannot sing' is not a matter of opinion. That is factually incorrect. That's like saying Kelly Clarkson cannot sing and William Hung can and trying to play that off as all just a matter of personal taste and opinion. There is both a subjectivity and objectivity to art. If there is no objectivity, everything is just as good as everything else and nothing can be evaluated relatively - which is obviously an obsurd notion if you follow that logic to its conclusion(s). I like to think I know at least a few things about pitch and range, and Kai is damn good with both. Consider the intro to Armageddon and the chorus of Heart of the Unicorn. That is an extremely expansive vocal range (and that's not falsetto BS or gutteral BS on either end). His pitch is dead on, he gets tons of emotional inflection into his lyrics, and he has an unrivaled (my opinion) skill at layering different types of vocal variations for a really arabesque sound. The whole conclusion of Beyond the Black Hole pretty much certifies Kai as an amazing vocalist - those are some extremely dense and clever harmonies. So, you can say you do not like Kai's singing, but you cannot say Kai cannot sing - unless you want to be wrong (not opinion wrong, just wrong wrong).