I have always had a soft spot for Shimazu, on account of them being the first clan I ever played on the original Shogun but rarely play them..
I find in S2 they get wiped out early the majority of the time and so I tend to bring them back when I get around to dealing with Ito..
As I am nearly always based in or around Kyoto at this time they provide an excellent and much needed stable trade partner after RD..
As far as Shimazu go I have not had them turn on me yet..
I have tried a similar thing with the Tokugawa but they are a lot less reliable, although I am reasonably sure I did have shared borders at the time of reviving them..and therefore took the diplomatic hit there..

Haketayama refuse to take Kyoto and I am one province away from RD so I am going to take Kyoto for myself and see the fun ensue from RD as I am vying for shogunism ;-)