How well internal cables work depends on how well the frame was designed. Some have no rubber seal where the FD cable exits the frame, on it's way up to the FD. That's right where the rear tire throws a stream of water. My Colnago has a tight fitting seal that should reduce water intrusion, but then I never ride in the rain (deliberately). Both cables have cable housing liner around the BB area, to keep out dirt.

Changing out the internal cables takes a lot longer. I looked over the Park Tool video on their cable guide kit and saw nothing of value in it, for my Colnago. If I lose the cable routing, a length of small diameter welding rod can be used to get through the holes at each end. I then use some heat shrink tubing to join the cable to the welding rod and pull the cable into the frame. It's still a real pain. I try to retain the pathway by using heat shrink tubing to join a guide cable to the old cable and pull it all the way up to the head tube, when I change a cable. Just don't let the guide cable slip out of the hole near the head tube, or it's back to probing with the welding rod.

I would not try to convert any frame over to internal.