You know, if you use "Sensei Slot" as a "thing that allows me to pursue an Alliance", you can make Mono-Clan decks ~Special~ by "freeing up" the Sensei Slot for them, allowing them something that allows them to potentially stay competetive. Basically, you either play a sensei (special ability empowering your mono claniness), or you play an alliance (ability to use different clan and maybe weaker special ability).
As you get all of the cards anyway, it's better to encourage people to try out different clans and decks instead of pidgeonholing them into clan loyalty, imho .
Mono clan decks shouldn't need an alliance to stay competitive either as it defeats the purpose behind having that clan to begin with. I haven't really heard anything about a disadvantage to these alliance cards.