Incidentally, Japan faces this very issue to this day; part of the country run on 50Hz, the rest on 60Hz.
This made matters trickier after Fukushima, as the nation is effectively two smaller electricity grids, not one large one - so making up for the shortfall became harder than it could have been. (However, there's a massive frequency converter interface between the two grids.)
Edit: Aw, shucks - now that I revisit the article, I see the exact same points being made in that article's comment section. My bad.
-It is not much of an issue on a micro scale - biggest issue today is probably direct-drive electrical motors, which will run too fast (if 50Hz on a 60Hz grid) or too slow (vice versa).
Also, running a 60Hz motor on a 50Hz grid will cause it to run hotter (larger current than nominally on 60Hz; also, less efficient cooling as it turns slower). If the designer designed to a price point rather than to a standard, this may be an issue.
On the macro scale, however, you've got a problem if, say, you suddenly lose a power plant in the 50Hz region of your country. You cannot then simply make up for the shortfall by distributing the load between all plants in the country (or, unless you're an island nation like Japan - from your neighbours, too) - only the ones in the 50Hz part of it.
This is just a long-winded way of saying that the larger your power grid, the more robust it is when a power plant goes offline.
The Japanese are in the unenviable position of having two small national grids rather than one large one - and, for an encore, they're on a bunch of islands and likely unable to import significant power from neighbours, too!
Hence any plant downtime is felt much harder than it would any other place. Tough luck.
This made matters trickier after Fukushima, as the nation is effectively two smaller electricity grids, not one large one - so making up for the shortfall became harder than it could have been. (However, there's a massive frequency converter interface between the two grids.)
Edit: Aw, shucks - now that I revisit the article, I see the exact same points being made in that article's comment section. My bad.