The concept of multiclass in its core seems good. However, due to the huge difference in car performance and player skill races end up in misery too many times.
The good thing about multi is it allows us to be competative with lower compounds which, in my opinion really adds a dimension in terms of preference (powerbuild or grip) and strategy/tirewear.
Id opt to narrow down multi to just 2 classes, permanent or in rotation. C and B and A and S. The performance will still be way off but the differences wont be as huge as they are now.