I was thinking the same, but really the advice hinges on two things, the first is the term 'unenforceable' and the second is the cost of litigation. In my case I went back and forth with my lawyer on whether or not their wording was enforceable in California and his advice was that given that California is a right to work state, and they explicitly remind you they can fire you for any reason, and disagreeing about ownership isn't protected by the right to work law (like discrimination is), if you're suing them you're not working for them (they will fire you), and if you really think there is a chance that something you might work on in your spare time could be "valuable" than the best thing to do would be to quit. Very unambiguous that.

I found it hard to argue with his logic.

It really depends on the jurisdiction. If very similar clauses have already been tested and thrown out in the appropriate court, then sure, you don't need to worry about them.

If a lawyer is advising you that something is unenforceable, they're probably right.