I had the IS on and was shooting at good shutter speeds...
RAW + JPEG, Aperture Priority, AWB, Center Weighted metering. ISO 100 - 400, f8-11.
What do you mean by 'good shutter speeds'? I was shooting in full afternoon sun yesterday, f/5.6 and ISO 400 for 1/1000 s, so in that light f/8-11 would be giving me 1/250 - 1/500 s shutter speeds - fine for static subjects, inadequate for cars in a drag race (too slow to freeze action, to fast for panning to show motion blur).
At f/8 with a fast enough shutter speed, you should certainly be getting sharp shots from the 6D + 24-105L combo, and if your subject was reasonably distant, the deep DoF should obviate mild misfocusing (which could be corrected with AFMA).
I don't know anything about the company you rented the equipment from. Personally, I know I can trust anything I rent from LensRentals, but I haven't dealt with other companies.
I've heard of borrowlenses.com, but have no experience with them. I would be tempted to use them, because they have a depot local to me which means no shipping costs. However, I know that lensrentals.com tests gear after it's returned from a rental, and adjusts/fixes it if necessary (lenses are tested on their optical bench). I also know that many rental outfits do not test their gear, but rather allow the next renter to discover a problem (not saying that borrowlenses does so, I have no idea whether or not they test the returned lenses beyond a basic, yes the AF and IS work, and it takes a picture).