AF speed and accuracy is a two-sided coin. The lens only responds to instructions from the camera, but the camera can only be as good as the image it receives from the lens. The Tamron's AF mechanism is actually pretty fast, no problem there.
The Tamron's difficulty, and the similar-spec Sigma 150-600 Sport, is the maximum aperture at 600mm is f/6.3. That is above the f/5.6 ceiling of most cameras. Note that Canon does not produce a lens with a max aperture higher than f/5.6 and if you go above that with an extender etc, the AF switches out, so lenses that run at f/6.3 have to fool the AF system into continuing to function. Only the 7D2, 5D3 and 1DX will AF up to f/8, but speed/accuracy is still compromised by the physically smaller aperture. It's the actual diameter of the aperture hole that's critical with phase-detect AF.
So at f/6.3 the lens is already fighting with one hand behind its back. If you then add into the equation a camera with less good AF, with less than optimum AF set-up, poor light and lack of a good contrasty target, plus an inexperienced long lens user (big lenses are not easy to get the best from) then you end up with stories about poor AF performance. Do the opposite - good camera, optimum AF set-up, centre-point AF accurately nailed to a nice contrasty area of the subject by someone who's had a bit of practise, and you get reports of very good AF performance.
Back to the Canon 100-400 Mk2 - that will always score a higher hit rate, particularly with less experienced long lens users. It's a top quality lens, much higher priced, it's f/5.6 at 400mm, and with only two-thirds the focal length of the Tamron and a lot less weight, and far easier to use.