Uncapped losses, it is considered the riskiest. Some brokers won't allow naked calls unless you can qualify for PM.
If you sel a covered call and then you sell the stock, now you are naked and most brokers do not have a way of identifying that so on non margin accounts it should not be allowed.
you can also sell covered call in cash account in IB but then you cannot sell the stocks before you buy back the calls and if the stock goes up, you might not have enough cash to buy back the calls.