The answer to your question depends on how hot you're running.
If I run about 215-225 at the grate, after 3 hours unfoiled, I don't find 1.5 hours in foil as being excessive.
However, if I'm running hotter, an hour can be just right.
You're looking at a combination of factors ... the size of the racks, grate temperature, time in foil, time out of foil, etc.. Add to that the fact that not everyone likes their ribs cooked the same way.