The primary difference between Rek'sai and Shen is that Rek'sai cannot have the global presence a Shen has, simply because the tunnel has to be placed and stay there before that global presence can happen. In order for Rek'sai to copy what Shen can do the Rek'sai in question would have to go into the jungle the enemy team will be near, create and tunnel, leave to the other side of the map, miraculously keep the tunnel there for the 2-3 minutes travel and splitpushing will do, and then ult down. Hoping there's no one nearby to poke her as she ults as it can interrupted easily. Meanwhile all Shen has to do is just walk top lane. Shen, yes, cannot push neutral objectives like that without teleport, but the fact is because his ults are always on allies, he will always be 'with' them even when he's not nearby when his ult is up. Meaning a 4v5 is almost guaranteed in favor of Shen's team unless someone makes a silly mistake.
Keep in mind this happens already with his current kit, but the rest of his kit is underpowered enough that he can't just keep pushing and he doesn't have as much of a teamfight presence as he could have/did before. Fixing that weaker point of his will bring him back over a tipping point, and his ult needs to have a fundamental mechanic change to bring him back to a healthy state.
So in all honesty I agree with you, he is underpowered and does not fight well as he is. However I also agree with that logic currently because of how is ultimate works.