I used to swear by Speedplay Zero Road pedals before I got into racing gravel and MTB. After that, it just made total sense to have shoes and pedals that can be used on all surfaces and all of my bikes. I still have some friends that use those 3 bolt SPDs and are always fidgeting with them, picking rocks out, or kvetching about squeaking and duck-walking
You don't necessarily have to switch shoes either. There are adapters to convert a 3 bolt shoe to a 2 bolt cleat.
Singlespeed MTB doesn't allow for a light touch on pedals on the hills, and there's no dancing up them like Contador. It's a gut-wrenching, muscle searing slog up steep pitches that are constantly fighting back with every bit of rock, grit, and inch of incline. I'd also point out that MTB pedals most likely experience more pulling on the upstroke than any road pedal, as inclines off-pavement are often steeper and more technical, thus requiring more effort on the upstroke (granted, I haven't found any specific study to validate this) I think it'd be difficult, nigh impossible, to find objective studies on pedal stroke efficiency that weren't from biased sources like pedal mfrs.
I don't recall where I read it but, other than in pure sprints and steep inclines; it's largely a myth that riders pull on the upstroke. It's more that riders will unload the rising crank arm, but maintain connection and begin engagement on that side as the pedal crests over the top.
My personal favorite on the road bike has become Ritchey's WCS cleats and their Road-WCS pedal. The Ritchey cleat works with all Shimano MTB pedals as well. (But not vise-versa.)