What kind of workload do you need to scale Synapse too? I mean, matrix.org is running on Synapse...
I know people who've been hosting the same XMPP server for decades. So this really shows the immaturity of Matrix. I still believe in it but it needs some more time.
No problems so far and performance has been steadily getting better. There was a low period when performance was not up to par about two years ago, but ever since that's been fixed, it's been smooth sailing.
Matrix.org has terrible performance, it's really a proof that synapse doesn't scale well.