If you get ISOs (not NQSOs) and you early-exercise them such that the spread is $0 (or at least negligible/low) and you correctly file an 83b and the IPO or exit is more than two years from date-of-grant and one year from date-of-exercise, then you can get the more favorable long term capital gains tax treatment on the generated income from the exit event.

That’s a lot of conditions, so IMO they don’t make all that much sense and I much prefer RSUs (maybe I’d think differently as a founder). Plus there are sharp edges like AMT. I’ve been an early employee multiple times at companies that had successful exits and never successfully had all conditions satisfied, and have ended up paying regular income tax rates but had more complicated taxes to file.

If joining a company as a non-founder I’d just take straight RSUs.