Comment by Liesmith

Of course it is, don't be naive. The people he's talking about are in a position to badmouth him right back, end even if they don't he can get a reputation for being a whiner who will turn on people who hire him the moment things don't work out. If you think that publicly criticizing your ex-employers isn't harmful to your future prospects then you're crazy. No one wants to hire someone who might turn around and complain about them to all and sundry if things go poorly.