Terrible, short-sighted leadership. Everything is a panic and a fire drill all the time. No work-life balance. Despite having unlimited PTO and working from home, the inept leaders will find a way to ruin all of that for you with their lack of planning and jumping from one thing to the next. If you're trying to work on a project that should take a few days anywhere else, at Upwork it either take weeks, months, or years to make any progress because some spineless "director" will come tell you to drop everything and work on something completely unrelated to your job.
It's always because the executive leadership team didn't have much of a plan, or couldn't communicate their plan coherently to the rest of the company, and middle management is mostly comprised of spineless yes-people, so it seems like everything is done on-the-fly.
Although the culture is generally okay, Upwork is run by a small clique of insiders. Your career advancement will plateau, or even turn sharply downward, if you don't get on the inside of that clique. Feels a lot like high school at times.