One of the big problems is that if you have all of your eggs in one basket, there are much more chances to fuck up and get banned. If I post some shit on YouTube, that is unconnected to my Fastmail account. However, it is connected to my Gmail account. If I get Zucked from Facebook for posting dumb shit, and dependent on Messenger to communicate with people, I'm fucked. By diversifying your compartmentalise your risk.