I use Signal as well and that’s what came to mind. Let’s assume I cheated on my wife and hired prostitutes using Signal on my phone. She could use my Windows PC, open Signal there, then see the cheater texts. This isn’t the fault of Signal, Apple, or Microsoft. It did the thing I asked it to do - sync messages. I would have fucked up by letting someone use my Windows login.
Good thing we don’t share accounts, aside from some very short term usage. That’s just a bad idea, even if it’s little personalization type things. Not messaging hookers probably goes a long way too.