Katyusha, Panzerwerfer and Walking Stuka are rocket artillery, which saturate an area with a lot of rockets. The first two lack the punch to tackle most even slightly armored units, while the latter is ineffective against single targets.
Priest is no rocket artillery, but something in the middle between the Howitzer and the leFH. It's price is higher than both of those, but considering that it's mobile and packs such a punch I think it's fine. It's definitely a unit that needs to be protected more and should be fielded later than the rocket artillery, but I can't see any problems performance wise.
Uh no, both the katyusha and panzerwerfer can damage tanks, especially the katyusha with the latest patch, walking stuka can bring any tank in the US arsenal on the brink of death except for the sherman.
The only thing the priest does better is damage heavy tanks and considering how slow it is we're only talking about jagdtiger and elefant.