For now, I am going to keep the factory installed X and Y mechanical switches on my mill. Like you saw in the pictures, the Z limit switches are completely enclosed in the quill housing where no chips go. On my Hardinge CHNC, the Z axis limit switch is on the side of the bed away from the spindle. I have never noticed any chips getting stuck to the magnet. Hardinge uses a much bigger magnet, about 1/2" in diameter so they can mount the Hall switch further away. If you had some dust that was magnetic, I could see that sticking to the magnet much easier. On my mill the mechanical limit switches are mounted under the table where I have not found any chips, but I don't mill anything the makes magnetic dust and I usually use coolant that would keep the really small particles (like when machining cast iron) out of the air.
Vince