I know you asked for people who went through it who regretted it. I have been through two one was prestigious and another was brand new, and loved both for different reasons.
You also forgot that most very popular incubators/accelerators also ask for some level of traction that is huge in and of themselves. 500 startups, for instance in the last batch had a bar that required reaching $100K Monthly recurring revenue or close. I know because we interviewed.
With that said, caveat emptor. Be very wary of what the goals are of anything you take part in and ask yourself selfishly what do we get out of this and be very explicit about that should you be accepted. Most of these programs are only as good as you are able to determine what you need. You must be forthright on making their goals align with yours and hustle to get what you need.