I've only killed one deer with it. Arrow zipped through and stuck into the ground. Deer ran 120 or less. The string was still attached to the arrow, which made two lines to follow, one red and one white.
Even if the line snaps at some point, it gives you a start at tracking, hopefully at least to a bloodtrail, if not the animal.
A deer may pull your entire spool out if not lethally hit, either snapping the string or pulling it all through his body. A bad hit is a bad hit. Nothing is guaranteed.
I missed two cleanly this season. One arrow stuck into the ground and I could easily see where it landed with the string going right to it. The other hit a rock and broke. I could see the feathers and thought the whole arrow was there. With the tracker, I was able to find my broad head in the leaves with 5" of arrow attached, 10-12' away.
I don't see a downside to using one, unless you take shots over 20 yards or try to shoot through brush/branches.