For the even good amateur rider, shoe fits matter more than road versus mtb style. Lots of roadies prefer mountain bike shoes and clips because they can walk around better in them. So hard to quantify a power difference. Theory goes that a stiffer sole shoe is more efficient. In practice however this difference is miniscule.
If you watched the GCN video, under steady state conditions there is virtually no power difference between platform pedals and more flexible shoes and road shoes clipped in. Sprinting and climbing out of the saddle is a very different matter and likely so is in my experience high cadence riding where foot security is a great premium. I like to clip in as a result if riding aggressively with peers. The rest of the time, the convenience of not clipping in is appreciated and casual riding in tennis shoes is comfy.