I'd probably be glad if the shortfalls were covered by something other than more stock with the same conditions and yet another 1-2 year vesting period (whereas almost every other company issues no-strings-attached cash bonuses). Remember that the bonuses are supposed to be a reward/bonus for past deeds, and delaying that sort of compensation by a year or two is not the way to make your top contributors feel valued.
The thing that started my exit was my final performance review. The previous year, I'd:
Unfortunately, the stock had taken a bit of beating early that year, so my RSUs (which by L6 are a pretty huge part of your total comp) were projecting to be about $20-25k short of my formal comp target that year.
Did they give me a cash bonus to make up the difference? No. Did they even give me an equivalent value in RSUs? No. They gave me about $12k worth (at the time of calculation) of RSUs that would vest over the following year. When I pointed out the discrepancy to my manager, he explained that the RSUs were supposed to appreciate by the end of the compensation year, so that by the time they actually vested, the new "bonus" RSUs (which were for the previous year, mind you) plus the RSUs I was already scheduled to receive should make up the $25k by the end of the year. I knew it was bullshit and he knew it was bullshit, but he was just a first line manager, a messenger, and a genuinely good guy otherwise (who fought hard to get me promoted), so what could he do?
Well, they didn't. The following year, there was no mention of this in my personal comp statement or perf review. Ah, well, guess the stock just came up short! I was poached three weeks later.
See, I know and understand that stock-based compensation has risks. That's fine, and that's why it's usually wielded as a motivational tool for really well compensated executives and as a bonus for ICs who do have some tangible power to drive the stock one way or another. I also understand that as an employee, rather than an employer (or the "owner of the means of production," in Marxist parlance) you're trading high risk and high rewards for (much) more modest rewards, lower risk, and additional stability. While my successes were worth many millions, Amazon gets the lion's share because they provided the facilities, equipment, and framework to launch those projects. That's how the normal employment model works--I get it, and I accept it.
The real problem here is that a great deal of additional risk (in this case) is still born by the employee, while the possible rewards are minimized as much as possible (via fixed comp targets with weak raises and re-up grants during banner years). Most people figure this out pretty quickly, which is yet another reason Amazon struggles with retention.