The Hummel and Grille's 150mm shells take forever to reload, meanwhile the enemy tanks are shooting back and going through their weak armour like butter.
The Wespe's 105mm shell loads slightly quicker but it too soon gets shot to pieces.
All three get a few powerful Hollow Charge shells (if they're lucky) but the trouble is their guns low muzzle velocities means they can't usually hit anything except at shorter ranges.
All that's why the Germans never used them for serious anti-tank work and developed the Marder instead.
The Nashorn sounds good on paper but its 88mm shell reloads slower than the Marder, also it's bigger and easier to hit by return fire, so i'll take Marders thank you very much because they're dirt cheap and can carve up Shermans and T-34's dead easy.