It's best not to sign them. You are signing away your rights to work on and talk about some things. The problem is that which things those are exactly is unknow

It's best not to sign them. You are signing away your rights to work on and talk about some things. The problem is that which things those are exactly is unknown to you at the time of signing. From the other side, as to whether to ask someone to sign an NDA, I would just refrain from disclosing key information instead. Way more effective. Or if you need to disclose in order to move forward, keep in mind execution is what really matters, and so you need to be really focused on being ahead, to the point that even someone getting hold of some of your ideas still can't keep up. These are just opinions... ymmv.