1. They released a "checking account" that they claimed was FDIC and/or SPIC insured when it was not. (yes, they backed off of this one when they got caught).
2. They've gamified buying/selling stocks using addictive dark patterns (like confetti explosions). FOMO-inspiring push notifications are another example here.
3. They have such a terrible level of support for a product which involves people's real money.
4. They let people start trading options without any of the due-diligence typically performed.
5. They released an initial options product which was quite literally "Do you think this stock is going to go UP or DOWN?"
6. Their entire product is based on people having margin accounts that do not know that they have margin accounts.
7. Their Payment for Order Flow is (or at least was) significantly higher than industry standard.
Overall, their business model is taking a variety of complex financial instruments and wrapping them up in lipstick to sell to people who overwhelmingly do not understand the thing they are actually buying (or selling).
I'm sure I'm forgetting a couple but I'm also multi-tasking at the moment.
Edit:
I forgot about the "infinite money" "glitch", which was a situation explicitly called out in regulation as prohibited (regarding counting outstanding margin credit as assets when calculating margin credit)