Is it safe to run disc brakes up front on a mountain bike that lacks a disc caliper mounting point in the rear? Upon sharing my thoughts in an answer to a Bicycle Stack Exchange question, I wondered about my suggestion to use a disc brake up front on a frame that, lacking a disc brake caliper mounting point at the rear and thus implying a lack of additional design considerations for disc brake usage in either the front or rear.  I've a lifetime bicycle tinkering experience, and I know I've heard of running a front disc brake though I can't remember actually seeing this setup.  So, given a bicycle, designed and marketed as a "mountain bike" but lacking a disc brake mounting point on the frame and also equipped with an after market, yet otherwise compatible, suspension fork which has both disc and rim brake mounts: would it be safe to run a disc brake (and rim, of course) in the front of this bike?  If not, why? Any considerations as to safety or viability between the two types of disc systems, hydraulic or cable-mechanical? (Above and beyond the lever, including the ability to use matching front and rear levers in a mechanical disc + v-brake set up).
A few years ago this was a reasonably common setup as sold. I've certainly ridden an e-bike set up that way, with v-brakes at the back and a mechanical disc brake at the front. I suspect this wasn't so much about stopping power as not needing maintenance between services, though in practice some adjustment was required even if less than at the back.