TL;DR: It might be a "red flag" sometimes (maybe even often) but not always. Some companies have good intentions with such a policy but their employees might not feel comfortable taking enough time off.
I am a manager, and was also an engineer, at a company with an unlimited PTO policy. I can't speak for all companies, but I can say that in my experience, "Unlimited PTO" at the company I work for is as good or better than PTO policies I have seen elsewhere in my career.
When my team requests PTO, my criteria for accepting or requesting changes is as follows:
- Who else has PTO that week -- does this request mean too many people will be out, and we'll be unable to handle an issue? (We're an operations team, for context)
- Is there anything during that PTO that the requestor needs to attend -- presentations, commitments, meetings, etc. Can they be moved, recorded, or, can we find a replacement?
- Are there deadlines or team commitments that would be at risk if this person takes time off at this time?
If there's concerns based on those criteria, then I would ask my employee to reschedule, not outright reject the PTO. And it's a conversation, not an outright denial.
For what it's worth: in over a year in my management role I have never needed to ask for anyone to adjust their PTO requests, and I have never been denied or asked to change my own PTO requests.
Note that none of my criteria includes "how much time has this individual taken so far?" -- This is not even a metric I track, because it's not data I need. Instead, at performance review time, I sit down with each of my team members one-on-one, and look at how they are performing against the goals we set for them. If people aren't meeting their goals, we can look at reasons. (And "too much PTO" has never been one of those reasons)
This is just one datapoint to answer your question "Is avoiding companies with Unlimited PTO a bad idea?". I would be disappointed if you did not consider one of the positions I am hiring for simply because we have an unlimited PTO policy.
The other answers make some very good points, specifically with regard to accrued PTO that, at other companies, must be "paid out" when leaving. In this regard, Unlimited PTO does benefit the company more than the employee.
After reading other answers on this question, and discussing internally, I'm going to start tracking last time PTO was taken by my team, and reminding/encouraging people on my team to take PTO if they haven't done so recently, to ensure that people don't feel like they shouldn't be taking PTO.