MERGE Engine vs. UNION ALL

We've been experimenting with the merge engine.

But suppose that instead of using the MERGE engine I instead modified my
code to UNION ALL the shards.

Would I get worse performance? In other words, besides the convenience,
does the MERGE engine have specific performance optimizations that make
it perform better?

This is of interest for several reasons. First, my code could determine
in some cases that only a few (or a single) shards were needed. Second,
I could apply the shard technique to InnoDB tables. Third, I could apply
the shard technique to databases that were not identical.

I didn't get any response in the MERGE forum.

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