I am always a bit sceptical about car covers. Most of the breathable external covers are not actually fully waterproof and after wet days, you will find wet areas on the car where the cover touches. Quite a few manufacturers now also recommend that their covers are not suitable for resprayed cars, due to the possibility of entrapped moisture causing micro blistering. Since most of our cars have been resprayed at some time, this is a potential worry.
I use one of the Halfords ones, after a Stormforce one degraded after two years intermittent use and Stormforce said that this was due to UV and quite normal….well this is Lancashire not Tenerife, and I felt that given the price new, this was not reasonable. The Halfords ones are a similar material but less expensive and seem as effective.
If my car has to sit outside with a cover for any length of time, I make sure that, weather permitting, it is removed every few days and the car allowed to air. Thus far this seems to have avoided any issues.