Article 50 gives all the negotiating power to the EU. It guarantees the ECJ a say, and possibly even the ECHR. It’s an expanded version of the secession clauses that were in the USSR’s constitutions.
It is not a foregone conclusion that EU courts would have jurisdiction over a rebus sic stantibus invocation, because that involves the Vienna conventions and UK sovereignty has to be respected in that case.
I'm told it does by people who are apparently experts. Short version as I understand it - Article 50 is fairly unambiguous (Any Member State may decide to withdraw from the Union in accordance with its own constitutional requirements.) whereas whether withdrawal under clausula rebus sic stantibus is highly debatable for two reasons - references to the EU as inherently permanent in what is now Article 53, and the fact that the UK would find it hard to argue that the possibility of 'changed circumstances' had not been contemplated, given the debates of the 1970s (where British politicians expressly stated that they worried the treaty would be broadened) - under the Vienna conventions, clausula rebus sic stantibus does not apply if the changes were contemplated and so should have been taken into account when the treaty was signed.