It's overwhelmingly more of an outsider talking point than an actual issue in practice. There's a category of people that just says any extensible protocol must fundamentally have massive amounts of incompatibility, and brings it up every chance they can... and while that is technically always possible, it only happens in practice if clients diverge greatly. XMPP clients mostly work together much better than Matrix clients, from what I've experienced, as long as they've been actively developed at some point in the last decade. Which is by far most clients in use.