I'd just like to reiterate this question from six months ago, as there was no answer. How do Ray- and Sphere- and WhatnotCast() treat mesh colliders?
My tests seem to suggest that the casts simply ignore mesh colliders. Mine was convex.
Raycasts do not ignore any colliders, mesh or otherwise, unless set up to do so with layers. They will "go through" the back sides of polygons, just like the back sides of polygons are (usually) invisible.
SphereCasts have trouble detecting objects that are within the radius around their origin, especially those behind the casting direction. In the case of my 2d game, I started the SphereCast X units above the gameplay plane and made it go X units below. That seems to catch everything properly.
I definitely consider this a bug and lost a lot of time to it. How do I report it so that others don't have to go through all of this?