Porsche gets high marks for usability, in a surprise to nobody.
Those who are in the market for an Alfa Romeo 4C won't necessarily be cross shopping with the Porsche Cayman. The little car faces more direct competition from the Lotus Elise. But Chris Harris has decided to compare it to the Cayman anyway, in order to highlight its issues being driven on British roads. But that doesn't mean he doesn't like the 4C, and neither car is exactly a clear winner over the other.
The 4C largely just suffers from the problem that basically every car has when being compared to a Porsche. It simply isn't as usable in the real world.