If you're trying to approach this simply as a mathematical problem (seems to be the case), then there is absolutely NO advantage in privileging monoclan. You're restricting available choices, reducing the appeal of the game for customers ( "we sell 200 cards, but you can only use 100 of them with your clan"), and forcing a certain play-style with each clan (something L5R players enjoy already, but others might not). Sure there are some advantages to monoclan, but they're not really mechanical.... they're mostly tied with playtesting (it is made simpler). And this is solved with a larger pool of playtesters.
However, this game, this IP, is not a simple mathematical problem. There are plenty of other objective and subjective factors to the game, and they have (arguably) much more influence than the mono/multiclan mechanical aspects of the CCG/LCG. So while it may be an interesting exercise to think in such restrictive scenario, you can't really take any conclusion from it, as the multiclan "question" was as important to L5R's IP in these 20 years, as a single grain of sand is to the Sahara.