Some territory that is today in Ukraine was formerly in Poland or the Austro-Hungarian province of Galicia. Currently, when you restrict the geographic scope of your search by changing "Any Place" to "Ukraine," results will NOT include directories classified under Poland or Galicia in the list at http://genealogyindexer.org/directories
. So, you should either use "Any Place" or also try "Poland" or "Galicia," which will find some Mayerowitz matches in places today in Ukraine (also, matches in places today in Poland). (Until recently, this was not an issue, since there was no "Ukraine" option due to a lack of Ukraine-specific sources.)
In the future, the "Ukraine" search will be improved to include all-Galicia and Eastern Galicia directories, but many directories with all-Poland coverage will likely not be shown for "Ukraine" searches, even if they include some territory now in Ukraine, because they are not organized geographically (e.g., they are organized first by occupation) and have no section with Ukraine-only content that can be easily separated.
A search restricted to "Russia" does not currently include sources that cover the entire Russian Empire, which included territory that is now in Ukraine, but instead is limited to sources with coverage contained in today's Russian Federation. The all-Russian Empire sources are currently classified geographically under "Multinational," but I might in the future add a "Russian Empire" option and rename "Russia" to "Russian Federation."
I realize this is a little confusing, but the scopes of historical sources do not match today's geopolitical boundaries. I will continue to simplify the search without introducing too many geographic false positives, but it will never be perfect due the non-geographical organizational structure of many directories. In the meantime, always at least try an "Any Place" search so that you see all results from all sources. If the number of results is too great, you can explore slices limited by date range (see http://genealogyindexer.org/advanced