A Google My Business Help thread has a local SEO who is furious with Google that over 60 of his clients were suspended from Google My Business (the local maps listings) overnight because he manages all these clients in a single account.
A Local Search Forum thread sums it up well:
A consultant had over 60 clients on one account, and they were all suspended overnight. In general, if a single business gets picked up for spam, any other businesses owned by the same Google account can get suspended as well. For those of you working in bulk with many locations on the same account, there might be other rules there, I'd be interested to hear if that's a danger for larger organizations as well. Either way, for those of you with just a lot of different clients, always make sure to keep them on their own designated account. I do that with all my clients for all other services anyway (hosting, domain registration, etc) since if/when they you two part ways it's nice not to have a bunch of annoying, time consuming work to give the client their logins.
It is similar to having your spam accounts in one Google Search Console account. Don't do it if you are spamming.
I honestly hate seeing this type of stuff where an SEO does something so dumb that all his clients suffer from it.