Yes Musk's track record is one factor, but mostly it's my personal experience seeing how poorly targeted Twitter's ads are. For example:

1. When I go on vacation I see ads in a language I don't understand

2. When I sign up for a service like Disney Plus I still see ads for that service

3. When I sign into a real estate website and mark some houses as 'favourite', that website doesn't show me ads on Twitter for similar houses

4. When I put an item in an online shopping cart and leave it there for a few days, I don't see any Twitter ads suggesting I complete the purchase

5. I live in an inner-city area, but for some reason Twitter shows me ads for combine harvesters and Starlink