Same gripe I've had on iOS past, could never figure out if my response was going to go over sms, imessage, or email, and if email which account, so I just avoid as much as possible, and don't setup additional accounts on my iOS devices or ones i set up. Mostly to avoid getting "why am i getting email responses to my text messages" questions etc.
Not saying it's hard or complicated, just hard and complicated enough for me to avoid. When I send someone a message there should be no guesswork involved. Not sure how both google and apple could mess this stuff up so badly in their attempts to make it "just work." Or how much of the issue is the "use only our ecosystem or your shit may to break" mentality.
:grumble grumble: