Mesh Union Issue?

Hello,

Having a weird issue with the Mesh Union vs Solid Union. 

I have a bunch of cubes in space.  They are spaced so that neighbors faces are matching.  With Solid Union, all the cubes are Unioned fine, though quite slowly.  I always used Mesh Union for such things, but somehow Mesh Union always seems to stop at a certain level, usually under the cubes.  In the image, the purple objects is the unioned cubes with Mesh Union, the black are the original cubes which union just fine with Solid Union...

This did not use to be the case, but I have not used Mesh Union in quite some time.  Any insight would be useful.  If you need me to supply files with geometry, let me know...