bunq does use some of its money that users deposited for Dutch mortgages. These are not 100% risk-free but in comparison they are quite safe and secure. There is also a bit of the money invested in green bonds. Most of the money is just parked on central bank accounts, though. Overall the risk of bunq going bankrupt and losing your deposited money should therefore be indeed a lot lower than at most other (big) banks. And then there‘s of course still the deposit guarantee that we as bunq users are protected by.
If you‘re interested in this, definitely check out all the documents provided here: https://www.bunq.com/policies/transparency and last but not least https://www.bunq.com/legal/climate-report this „Climate Report“ actually has a lot of information about how the deposited money is used 👍