Gambling is only bad thing if the odds are not in your favor.
Once it an "obvious" choice to join a startup then the valuation of the company is already close to fair, assuming you have about the same edge as VCs do when evaluating these companies. On IPO day it may jump up a bit from its last posted private valuation, but keep in mind once the company IPOs you're typically subjected to a 1-year lockup period, during which the value of the company could change drastically.
Getting the train moving is the hard part but when it is already going it’s about not screwing up and execution, many times better odds (of course evaluate it like you would evaluate stock before buying)
The number of X-s for large cap RSU-s are less likely to be amazing so joining those RSU-s should be pretty much considered at face value (or maybe 10% YoY)