The main problem is that the overwhelming majority only plays the best class combination available to them. One combination will always stick out and thus will be played.
Sadly many nerfs where directed to Champion in general, instead of nerfing the combinations specifically that were overperforming, which left certain secondary classes in a bad shape.
Moreover, Ch/W and Ch/P are two underwhelming combinations that just don't see play. Ch/W is a bad tank who nobody in his right mind would choose over Ch/K. Same with Ch/P, not really viable as a dps compared to other combinations and heavily outclassed as both tank and supporter by /K and /D respectively. Those two are in dire need of a rework in my opinion. Ideally one of the two should be made a dps, to increase the amount of combinations a Champion can choose from as a damage dealer.
We tried CH/p as dps before the last patch and it was nearly as strong as CH/m. Should be equal now. The main problem I saw is the aggro in trash phases of CH/p. I would say nearly no Knight can hold the aggro. If the CH/p skills wouldn't generate aggro in demo mode, it would be a fine class in my observations
Interesting, maybe the 30% patch made this class more viable. However, I'm quite positive it can't match Ch/M. Gonna test this myself over the next days.
Still, Ch/P is not a well rounded combination in my opinion. The rotation feels clunky, Reviving Blast is useless and the elite skills in general are all over the place, mixing dps, support and tank abilities. I'd wish for are more clear line regarding the role of Ch/P.