Thesis Vitaṇḍā is not always illegitimate: a critic with no thesis can still be doing philosophy
Where it stands
There is one problem in Nāgārjuna I have been chewing on for a long time.
The Nyāya tradition calls vitaṇḍā illegitimate: one party only attacks and never defends a position, so they risk nothing and cannot be wrong. That seems fair as a rule for a debate.
But Nāgārjuna's opponents accused him of exactly this, and he did not exactly deny it. In his Vigrahavyāvartanī he says, roughly, that if he had a thesis the fault would apply to him, but since he has none, it doesn't. The sceptic Jayarāśi, usually counted among the Cārvākas, wrote a whole book attacking every means of knowledge without offering one of his own. Sextus Empiricus did something similar in Greek. And Socrates, in the early dialogues, mostly refutes and says he doesn't know.
These people are not cheats. Some of them are among the most important philosophers anywhere.
My thesis: pure critique without a counter-position is a legitimate philosophical activity, and sometimes the most valuable one. What makes it illegitimate is not the lack of a thesis but pretending to be doing something else.