The bubble being green and Apple making them progressively uglier and breaking their own interface guides is one issue, but the main issue people care about are how SMS conversations break with an iPhone. Group threads will randomly have messages delivered in different threads, pictures and videos are low res if they send at all and there's no advanced features like typing indicators, read receipts, etc.

The hope is with RCS that is fixed. If so, the color of the bubbles doesn't really matter.