If we want to talk about technical issues, though, it's pretty stark. MMS delivery and message ordering is significantly less reliable than pretty much any OTT service out there. Message size limitations mean videos get recompressed to garbage when you send them. There are basically no add-on features, like reacting with emoji to messages, for example.
The disappointing thing is that there's a solution to all of this: RCS[0]. Android supports it, but Apple of course loves their iMessage walled garden and refuses to, even though RCS has end-to-end encryption now. iMessage may be technically superior in some ways, and RCS certainly has its issues, but personally I feel interop is much more important here.
Just send your fucking messages on your mobile devices and be done with it.