These US healthcare systems are effectively scams. Yes in theory they can save you money, however in theory there is no difference between theory and practice, while in practice there is.
It’s the analog to how the industry lobbies to get preferential tax treatments for these accounts. 99% of the time your employer or the insurer keeps money at the end of the year. If you’re smart about when you leave your employer though(leave after exhausting your FSA early in the year), or have advance notice of it, you can walk away and beat the game.