A proper rain cape is useless without mudguards, should fit closely at the neck, and have enough substance that it does not cling to you.
In the old days they were usually worn with southwesters so there was no problem with rain channelling down your neck – but that applies to any rainwear. You can get them with a hood which solves the problem.
They are brilliant because they are a lot more breathable than just about anything else. Another advantage is you can leave putting it on until the last minute. Good when it’s really cold and sleeting because you are in a bubble of warm air.
Best suited to a more upright riding style.
High winds are an obvious problem, but not insurmountable, and you’re not going to win the TdF in one.
If you want to get somewhere in ordinary clothes they are unbeatable.
Needless to say, there’s usually one lashed to my bike.
BTW I never noticed rain jackets being called capes until the last few years.