From my experience nothing has ever been "broken" or the like (at least after 1st/2nd update of a game), but it has always been a question of individually balancing the game world.
It's a fact that only playable leagues produce active clubs when it comes to transfers. If you have just 1-3 playable first divisions loaded and a certain/huge amount of players, there will be no balanced market for players, obviously. It's a simple question of supply and demand: many players and few (active) clubs = no or little demand and low prices. The other way around: many clubs and few(er) players = high/decent demand and higher prices.
Having the first divisions of the top 5-7 nations in Europe playable and in addition neighbouring countries depending on the country you want to manage in will always result in a quite active transfermarket. If your computer can't cope with 10 playable leagues and/or the game speed becomes too slow (which is obviously a very subjective perception), one should reduce the amount of players.
For me personally, the above has always proven true, although there is no strict rule of how to setup a game (such as a scale or the like for the relation of active clubs and players in the game). It's a bit of testing and experience. I would say, definitely avoid the extremes (see above). However, I think that these basics about how the game world is working should be communicated more transparently by SI itself. But I also understand that they don't ;-)
By the way, having the top leagues loaded as playable will also result in stronger opponents in the European competitions ;-)