I don't find it underwhelming as much as I find details in the execution as flawed, and the rational underpinnings of the series are in total contradiction to the story of the finale.
1) Make up was inconsistent. They did an awesome initial job of making both of them age in appearance. But proper makeup artists would have mapped the face so they could recreate the wrinkles on multiple shoots. I've read people speculate that they wanted them both to look younger as the episode progressed, as if they're looking at each other from their eyes. Ultimately, it just looks like a lazy makeup job.
2) How the heck does Kevin die multiple times, but at 50-60 years old, suffers a heart attack, almost dies, and requires a pacemaker to exist? The show was premised on the non-intervention of an omnipotent deity. The last episode is almost framed as a science fiction story. A physical phenomenon occurred, which separated people into different multi-universes, which was reproducible. How do you go from that to an implied divine hand once made Kevin Garvey immortal, and then took it away at 50-60 years of age, and no explanation of "why" or "why take it back"? Its so irrational and contradictory, it sort of obliterates any rational underpinning they used to explain the series' "reality".
3) I get that Nora is so emotionally devastated by the events after she goes into the alternate universe, that she doesn't attempt to contact Kevin. But she starts a pseudo contractual relationship with Laurie, yet never asks her (or Laurie providing an answer) "how Kevin is doing"????
The "rules" governing conduct for a therapist mostly legal in nature, and ethical in principle. In what court is Laurie going to be sued if she reveals to Nora that Kevin is "available for booty calls"??? Given that the legal preconditions are irrelevant, only the ethical considerations are relevant. What is ethical about refusing to provide information to Nora about Kevin's relationship status? Laurie and Kevin must exchange enough personal information during weddings and other family related events to get an idea that Kevin hasn't gotten over Nora, or at least isn't seeing someone else. This wall of nondisclosure only makes sense if they hated each other, and went out of their way not to exchange life details. And why would Laurie care about "therapist" patient rules, when she deliberately perpetrated a fraud (and violated therapy rules) to provide "therapy" via fake seances??? Kevin and Nora could have reunited at least 10 years earlier if Nora did the minimum due diligence, and Laurie passed over a scintilla of information.
And again, I get that Nora was emotionally devastated by the events after her transfer to the other universe. But she was willing to do that to tangibly reunite with her kids (including abandoning Kevin), but not willing to do baby steps for Kevin when she had nothing to lose???
The last episode is absurdities thrown together just to get an emotional payoff. But for me, they are just too incongruous and contradictory to allow "suspension of disbelief".