After doing basic napkin math I've always thought it was pretty obvious. Buy-in + annual maintenance fees that are pretty close to what a condo rental outside of a contract would be. Then you add in the other factors such that you're locked into certain locations and whatever timeframe is not booked, don't actually own anything other than a lease, and the only way to get out of it is to sell it yourself... no thank you.

They aren't an investment, period. You generally can't sell them. The complex can assess whatever maintainance fee they wish with little to no requirements for justification.