It depends. I haven't exactly rescued a startup, but worked on several completely failed products to bring them up to "workable" status for a lot of large companies.

Usually, if I'm working directly for the company, it is great. They understand the previous team failed and they want to move forward. It might be a little tough to ultimately gain their trust, but confidence goes a long way.

If I'm working as a subcontractor for the contractor/team that failed, it won't be fun (in my experience). They are not grateful, they are going to blow budget (their problem, but they make it yours) and they will blame you when the time comes for it. Also, they take forever to pay.

As long as you get paid in money rather than stock, and you cash the checks the minute they arrive.

The important par is "as a contractor" (with clearly defined exit conditions). Also, some projects are beyond rescue.