Even now, maybe not. Does anyone know whether most of these companies ever did share buybacks for the rank and file employees? If not, then imagine the scenario where you left somewhere between 2015 and 2019 and had 30(?) days to exercise ISOs. You might have to put in a ton of money and even more for the tax bill. Maybe more than you can afford.
I’d love to know whether this is a common story or a rare one, but I doubt that data exists publicly.
I had a company offer to arrange a purchase of some of my (non-public) stock as I was leaving. They knew of an investor that wanted to own more shares and brokered a price that was well above the most recent 409a, so I was able to exercise everything and sell off exactly enough to cover the tax consequences of my "gain." Not only did it let me exercise options to get stock without having to come up with a bunch of cash, but it gave me more confidence that I actually wanted the stock. Very cool move on the company's part, in my opinion. I have no idea how common this is.
Nowadays there are non-recourse loans available on the market for that (covering both exercise costs and taxes). Not for all startups though and mostly late stage ones.
I’d love to know whether this is a common story or a rare one, but I doubt that data exists publicly.