Really small can be an issue when it gets really small e.g. tarsnap’s accounting is pretty infamously done in attodollars (1e-12). U64 only has room for 19 decimal digits so you’re limited to 7 figures up.
Tarsnap's pricing is in picodollars (1e-12) but the accounting is attodollars (1e-18). Tarsnap uses 128-bit balances -- 64 bits of attodollars and 64 bits of whole dollars.
If I ever have someone paying me more than 2^64 dollars, I'll rewrite Tarsnap's accounting system.
