Wither roses affect mobs and players in a very weak way. The damage received is pretty low and it takes quite a while to kill even the weakest mobs.
What I expected:
I expected the Wither Rose to act as stated on the Wiki, "Wither roses inflict the Wither status effect to any players or non-immune mobs touching it, dealing 1♥ damage every half second." However, I believe this is not currently depicted properly.
Another way to put it, I expected the wither rose to kill mobs as fast as a magma block would.
How to reproduce:
Place a wither rose on the ground and place a mob in the same block space. It should take longer for said mob to die than on magma.
Screenshots Included: Yes
Normal behavior: https://www.youtube.com/watch?v=gTagpmpgC5o (Credits to wattles for the video)