Yes, it's very complicated, and that's a testimony to the depth of confusion regarding the Church today.
SSPX believes that Vatican II taught some errors. Archbishop Lefebvre, who signed off on all the V2 docs, minimized the problems with V2 as ambiguities, which could be interpreted in either a Catholic sense or a modernist sense, deliberately planted there by the modernists themselves, with the intent to exploit them down the road. I believe that most SSPX clergy and faithful would go further and say that there are in fact clear errors in V2. But Bishop Fellay appears to fall more into the "V2 ambiguity" school of thought, which attitude of his has been driving the recent talks with the Vatican. Archbishop Lefebvre told John Paul II that he would accept Vatican II if interpreted in the light of Tradition. JP2 responded something like, "But of course we have to understand it in the light of Tradition." As it turned out, JP2 had an evolution of dogma "heremeneutic" with regard to V2, and so that "agreement" almost immediately evaporated. Unfortunately, quasi-official-SSPX-theologian Michael Davies, for instance, reduced the error in V2 that could not be reconciled with Tradition (albeit with serious theological gymnastics) to the word "publicly" in Dignitatis Humanae. SSPX unfortunately does not understand that the errors and bad fruits of Vatican II derive from a completely new ecclesiology that revolves around a watered-down view of the dogma Extra Ecclesiam Nulla Salus--the same watered-down if not grossly erroneous and even implicitly heretical view of it that quasi-official SSPX publications have promoted.
Now I've used the term "quasi-official" with regard to SSPX, because there's hardly a single monolithic SSPX. Bishops Tissier, Williamson, and Galaretta are more hardline than Bishop Fellay, and among the SSPX priests and faithful you'll find anywhere from quasi-modernists to closet sedevacantists.
Now, the SSPX also holds that the NO Mass is intrinsically valid (or capable of being valid) but often in practice invalid due to a lack of proper intention from the priest. SSPX holds the NO Mass to be quasi-Protestant and harmful to faith. Yet they argue that this does not violate the theological certainty of disciplinary infallibility because it was never strictly, officially, legally mandated upon all the faithful by the V2 popes. It's a rather tortured legalistic argument. SSPX knows that many or most of the faithful want to have Sacraments received in the NO conditionally repeated (such as Confirmation), so they placate the masses by this theology of the bad intention.
So if you hold an ambiguity view of V2 and a non-legally-binding view of the NO Mass, you can posit that the V2 popes are legitimate. Some of the SSPX (including Bishop Fellay) are adamant that the V2 popes MUST be regarded as legitimate. Others, including Bishops Tissier, Williamson, and Galaretta have said that it's OK to entertain certain private doubts regarding the legitimacy of the V2 popes but that you can't get dogmatic about it. These same bishops would probably reject the ambiguity view of V2 as well.
SSPX holds that V2 and the V2 popes did not teach any of their errors infallibly--which opens up another huge discussion for which I don't have time at the moment.
So that's SSPX in a nutshell. I'll write later about sedevantism and its various flavors--and that gets even much more complicated.