Jon, I basically agree with you. But whereas those CSS deficiencies are typically invisible, or at least inconspicuous, to the naked/non-expert eye, missing or buggy functionality – such as no option to sync annotations in Marvin, or the corruption of source text when exporting annotations in every e-reader app I've yet encountered – is glaringly obvious to everyone.
So, yes: Marvin is very good, and so is MapleRead on iOS and Moon+ Reader on Android – but none of these apps is truly "great", or "5-star software", or "professional-grade" software (whichever epithet one might prefer).
