I have spent a lot of time on this subject of interest.
They have been tried, rotating cylinders, and a whole lot of other stuff.
This was all during WW2
YouTube has a bunch of information stored there too.
There are people trying to make these work to this hour.
AFAIK, or understand from porting projects, there is CFM flow and velocity.
There are all sorts of tricks to make common valves work better.
But the velocity is truly a work of art on the short side of the port (the curve.)
Remove that, and you get a slug.
There are engines running with these rotary valves of sorts..
But they dont work well.
As far as opposing pistons,
afaik this is in progress and may be working somewhere.
What we really need is a sort of Wankel that works.
There are several versions in progress.