If I recall correctly, if some node desyncs, it's hard to get it started again. You have to manually transfer the entire database from another node which can obviously take a long time if you have a large database. It doesn't really handle netsplits by itself (there's https://github.com/uwiger/unsplit) so you have to be prepared to do this yourself.
I had a _really_ bad time using it. I used it for a relatively straightforward application but boy did I regret it.
It's not intuitive at all and configuring it is a PITA. While there are docs, they are as murky as bog water if you're new. Deployment? My god, it had me tearing my hair out and I should have just ditched it all right then and there. Migration? AHAHAahahahah...
And my experience was on a SINGLE node. I struggle to comprehend the herculean effort of getting multi-node to work. And for what? It does not offer anything compared to either Redis, SQL, or anything that is not mnesia aside from "you don't need to depend on an external service".
YMMV of course, but IMO it was absolutely not worth the effort. Yes, I understand people might feel the same way towards Elixir and Phoenix, but at least in that case there are incentives such as an active community and ecosystem. No such thing with mnesia.