The tricky thing with these licenses (BSL, SSPL, etc.) is that you can use them freely for internal stuff, but suddenly, if you make your product public (assuming it uses, e.g., TimescaleDB), things can get muddy. Everyone wants the flexibility to either open-source or commercialize a successful internal product in the future.

The problem is that, even if your app is not a mere frontend for TimescaleDB/Mongo/Redis, you can get sued, and you'll have to spend unnecessary time and money proving things in court. No one wants this, especially a startup owner whose money and time are tight. Also, even if your startup/company uses some of these techs, potential company buyers will be very wary of the purchase if they know they'll have to deal with this later.