It's been said that the SSPX acts as a barometer for the Catholic Church. The further Her leaders take Her away from the truth, the more glaring the differences between the practice of the SSPX and that of the 'Modern' Church.
The first interesting article was this one from 1P5 ( 1P5: FSSP superior distinguishes fraternity from sspx eschews traditionalist label ).
As an aside I was greatly amused when a 'resistance' website made the mistake of thinking that this was an SSPX superior.
Anyway, it shows that at least in Germany there are significant differences between the FSSP and FSSPX.
I suspect that when the FSSPX is given the canonical standing that it deserves there will be a split within the FSSP.
Next we have this article ( Common Wealmagazine: Catholic Traditionalism Old and New) from Common Weal.