A conventionally well-designed site is actually much less trustworthy for me.

GNU cash website immediately tells that it is not a Saas and doesn't need to upsell the latest trendy addons, for it to survive.

It tells that it is not "investing" in marketing to eventually turn a profit.

It is not looking for acquisition opportunities or next funding rounds.

If you want to see what a trustworthy website looks like, take a look at SQLite or postgresql or even this website itself.