This test takes a pipeline with a supported $sum accumulator, which will be eligible for pushdown into sbe.

testPipeline([{$group: {_id: "$groupKey", sum: {$sum: "$val"}}}]);

Then the test compares the explain between running the pipeline alone and running the pipeline as an inner pipeline of $unionWith. We don't bother to pushdown $group when it's inside $unionWith, so naturally the explain plans are different. This work should figure out what the right update is for the test.