Not very in the grand scheme of things. For one, we have key eye witness testimony of some of the last occupants of the bunker. Heinz Linge (Hitlers Valet) and Otto Gunsche (Hitlers personal adjutant) were both present at Hitlers suicide-albeit he and Eva Braun committed suicide behind closed doors. They disposed or at least tried to dispose of his corpse. A lot of the hoopla surrounding Hitlers death seems to come from Stalins claim to Truman following the war that they weren't sure of his whereabouts, even though the NKVD had taken possession of his and Eva Brauns corpse. I think it is highly unlikely that Hitler could have successfully escaped (to Argentina?) and lived out the rest of his life in solitude.

Ever heard of Adolf Eichmann? Sure he was a key player in the Holocaust, but in the hierarchy of Nazi Germany he was a relatively minor official(a Lieutenant Colonel in the SS). The effort put into capturing Eichmann was considerable, now imagine a Hitler on the run and the effort that would be put into finding him. Hitler had made it plain to many that he would not be leaving Berlin and indeed he died there.

Sources : The Hitler Book: The Secret Dossier Prepared for Stalin from the Interrogations of Otto Guensche and Heinze Linge, Hitler's Closest Personal Aides.

The problem with figuring out whether this is likely or not is that historians are historians and anything that isn't generally accepted is basically a conspiracy theory. If some new idea were to gain traction, you'd hear it from investigators, not from historians.

The idea of Nazi party members being moved to Spain via the methods mentioned in the show is too obvious to not be practical. The trip across the Atlantic is a little far-fetched but if I'm able to track toward an RDF beacon and land a plane in that field, you can bet German pilots could as well.