I've shot a couple deer, 2 javelinas, a hog and several turkeys with string trackers. Worked pretty well overall, I consider almost necessity with turks. Never had an animal react to the noise IMO.
The trouble is, though, that the string seems to generally get hung up / break by 100 yds or so. Works fine with a good hit, but marginal hits I'd be worried you'll be in the same situation as no string after the first bit.
I don't use them anymore except turkeys.
We (the wife and I) are currently training a dachshund pup to blood trail - that's my solution to color blindness
R