https://www.cnet.com/roadshow/news/radical-new-engines-make-a-run-at-reality/
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DARPA, TARDEC, and some other BIG name companies have been working on the "flat opposing piston designs" for years...with the goal of High specific output from a package that can be easily integrated into existing and new designs for combat and combat support vehicles. They are also looking to make them modular, in the sense that if you need more power, just add another unit to the common drive shaft to increase capability. Downside is always cost, tooling, and complexity of the systems. They generally provide good power in a small package but longevity/reliability has been an issue. Achates developed the motor in the CNET article in cooperation with CUMMINS.
https://jalopnik.com/379440/tardec-opposed-piston-opposed-cylinder-2-cycle-diesel-engine (https://jalopnik.com/379440/tardec-opposed-piston-opposed-cylinder-2-cycle-diesel-engine)
https://cafe.foundation/v2/pdf_tech/MPG.engines/PAV.Engines.OPOC.NASAGlenn.pdf (https://cafe.foundation/v2/pdf_tech/MPG.engines/PAV.Engines.OPOC.NASAGlenn.pdf)