If it's against opponents that crawl across the ground then the quagmire has a tremendous effect, but if it's like a Griffin or Succubus, which can fly through the sky, then the effect is reduced by half.
For the insect like ones which are slow but have hard outer shells as well, even if you can stop their legs doesn't mean it will go well.
In the first place, recently it hasn't even been able to slow them down.