[Illuminated One]

The problem of Yomons diversity is that it's so random.
Sure, Yomon makes one of the best sitesearchers in the game, but one of the worst combatants and booster forgers imo.
Sure there are other very random mages but the nations that have them have either someone else to base their strategy on or they are enchantresses (which pretty much means that your sitesearching advantage is an illusion as some power nation will luck into them).
Yomon has only these. So, you start the game at turn 1 and start researching into conj3 and evo5 for GfH and Falling Fires. Let's say you have it with 25 mages. The chance that you have not a single mage able to cast these spells on his own is ~ 20% for each (the chance that you can get there with a communion is ~ 98% - I did ignore that the chances are not independent for the spells, i.e. if you have no GfH caster you have a better chance to but this shouldn't make a too big difference I think). Now maybe these values look not so bad but in comparison with other nations who can get their spells without communions always and ever, or with a very high probability (the chance that Tien Chi has 4 or more Rain of Stones casters in 20 Ancestor Smiths to kill a lot your communions is ~ 80%). Now, you also have the bad mapmove which means you either teleport around a lot or don't have the mages that can cast the spells that you went for where you need them.
In any case I've yet to see a game where Yomon does more than defend it's territory, so a boost certainly wouldn't hurt. I disagree about giving them d or b though.