The problem with riding SPD/clipless pedals with trainers is they are narrow so you’re pushing down on the inside half of your food. On my MTB i’ve got some flats with clips – Time Z control. Perfectly fine with trainers, reasonably grippy even though the clip bars protrude up form the surface of the pedal, but they just sink into the soft sole of the trainers. Unfortunately Z controls don’t seem to be available any more and the new Time platform style pedal that seems to have replaced them don’t look as good.
Opens up the debate about the point of road specific cleats though. I do have them on my road bike but struggling to notice any benefit over MTB style cleats. I suspect for the average person they’re point less and the average person would be better of using MTB style cleats on their road bikes too.