FSA has strict rules on reimbursement and timing. HSA doesn't (this post accurate as of 3/27/2021).
I actually learned an interesting strategy at work. I don't use it yet, but I plan to after I catch up on a couple things.
Our HSAs at work are investable, so some of our folks (keep in mind, we're almost to a person tax people) actually pay their medical out of pocket, while continuing to keep their HSA invested earning in the market. They keep track of their expenses on a separate system.
If they ever need a windfall for some reason (replace a roof, disaster, etc), they can turn in all their accrued expenses and get a tax free windfall. Meanwhile, the market earnings inside the HSA continue to be shielded from tax in the interim. Staying tax free forever as long as you always use it for medical.
