That's an interesting rabbit hole. Some FSA types, but not all, have full funds available on day 1 of the year. And no provision for clawback if you terminate your employment before the end of the year. So when you forfeit due to overestimating the total, it does not go to the IRS, but your company. But the upside is the funds being available on day 1, with you having to wait until the next tax filing rolls around.
Unless you're planning on quitting jobs mid year every year I wouldn't really call that an advantage. On all other years you either end up buying piles of shit you don't need just to not lose your FSA dollars you gambled that you would use but didn't, or end up underestimating your medical bills. It's such a broken system that one even has to make guesses like this. There is no need to make people guess.
> funds being available on day 1
Most people with FSAs have higher disposable income so this isn't really an advantage either. Lower income folks typically have HSAs instead.