I'm not sure the % but a high percentage of models I grab from thingiverse are unprintable and non manifold, or have gaps or weird holes. Usually https://tools3d.azurewebsites.net/ fixes it up for me or meshmixer. But what are other people using that they don't see these issues when slicing? I ALWAYS find the issue in simplify3d. This model https://www.thingiverse.com/thing:2457756, I just tried to print for someone at work has a hole in the switch logo...which when slicing caused the side parts to not be attached to the cup part .. other than that it looked ok when slicing. I opened it in mesh mixer and ran the inspector and it appears to have a hole somewhere in the switch logo on the front. I tried to print a storm trooper off thingiverse that had a gap in the layers half way through the chest...among other similar problems on many models. Just a rant, Im usually able to fix it but wtf? how are others not finding these problems. I almost always run my STL through that net fixer for the hell of it before uploading to thingiverse. Most of these models I find with these problems have pictures of printed ones, and other people that made them.. So whatever slicer they must use must ignore the problems? Simplify3d doesn't ignore them for me and I end up with messed up prints, or if I am lucky i notice it in the preview.