Dilution: there is a “dilution” row that lets you specify the dilution percentage for potential future rounds/valuations. I used 20% as the default because this is roughly what you'd expect in a typical priced round, but this is user-editable.
Tax: tax modeling is hard. I haven’t attempted to do it in this version. I might add it in future versions.
Fractional share rounding during monthly vesting: it rounds the running total of shares vested to date each month, then reports each month’s vested amount as the difference between this month’s rounded total and last month’s. This way, rounding errors never accumulate and the schedule always sums exactly to the full grant.
I’m not sure what you mean by “any incentives besides having a userbase?” Are you asking why I built it? I’m a startup GC and I do this type of modeling all the time, so I wanted to build the definitive tool, and I wanted to open-source it and share it with HN for feedback.