hbrednek Park Sider (as usual!) pretty much accurately broke it down for you. I love Collagen because it is easier to use than Hog or Sheep casings, no prep is needed and I also like the snap from a fresh collagen more than i like the snap from a hog casing. What I don’t like is exactly what your issue was with linking, they just don’t stay as tightly wrapped, one thing you can do that will help is after you link them and before you cut them put them in a freezer for about an hour. They will still have a tendency to come undone but they will hold a little better.
As for overstuffing it is a “feel” thing. You are most likely using too much pressure when holding the casing on the tube. A slightly understuffed casing is preferable in my opinion as you can simply twist it a few extra times and it will firm up where as an overstuffed casing will pop when you go to twist it.