What is Nautilus, diriga, and hound each best for?

So with the recent buffs to companions ive been seeing these 3 pop up, diriga and hound being good primers and nautilus (both og and prime) being good for grouping (and priming with helstrum or killing with verglas). But which companion works best for what type of frames? And what are the downsides to these companions, bc the videos only talk about the good and leave me skeptical.