Hi, welcome to OT.

What I've learnt is that orchids are like people, they may be the same nationality but that doesn't mean they're the same. I have 2 UFO onc and they're different in many ways. You may have to tweak your potting mix to be more fast drying for the sick ones, or they may need a bit more sun (keep using the humidity tray). Take them out of the pots and let them dry out for a day or so after you slice and dice the roots. Cinnamon, hydrogen peroxide, weak bleach water, SA20 or Physan are a few suggestions for cleaning up the cuts, to prevent fungus and bacteria infection and such, then repot. All orchids are trial and error unfortunately so you have to see how they do with the changes. If the roots are really bad and you're worried that they won't recover, don't, because they come back from the dead, dust with a bit of rooting compound as a booster before repotting and give a weak fert after a week so it settles and the rooting comp has chance to work into it. I have a "Death Door" shelf (there several threads in Ailments/Compost Pile about resurrection plants)and they always come back, unless they've really really gone south. and keep us posted.