I have some experience with a "use it or lose it" place.My wife's is use it or lose it by her anniversary date every year. This at least keeps everyone from trying to burn PTO at the same time and during the holidays each year. But still, she holds a couple days as " just in case " that need to be spent that last week. She works under a bargained agreement. So don't anyone start your " a union would stop that " .
The policy works as long as there is staffing to cover time off. Large nationwide , global company. The policy was altered periodically ( in favor of the employees ) when there was a shortage of people to cover vacation time and it was impossible to schedule time off.
At XPO Logistics , some locations ( a lot of locations? ) don't have the people to cover time off adequately so time off may be blocked. That sets up a "get a lawyer" scenario to me if it is a "use it or lose it" deal but you aren't allowed to use it.