"...stories are successful only if the audience understands, can follow, and is invested..." I would argue that people had no trouble following or understanding HIMYM, but they ran into problems because they ceased to be "invested" in the relationship between Ted and Robin. The couple had all ready formally dated twice, lived together, and enjoyed a casual sexual relationship. By the time the show ended, people were sick of the whole "Will they/Won't they?" conflict.
People were invested in seeing how Ted would behave with the mother in comparison to his other relationships. They wanted to see him shed some of that desperate, childish, manipulative behavior that ended up ruining all of his previous relationships. They felt short-changed when not only did they not get to see him with the mother in any meaningful way, but the last shot of the show is Ted outside Robin's building, holding a blue french horn, which is the exact same romantic gesture he did in the very first episode, showing that he didn't really change at all.
Now compare that to Barney's ending with the baby. Nobody talks about that. Because it was perfect. Predictable? Sure. But excellent. Believable. And it shows that Barney underwent a true internal change throughout the course of the show. The show is supposed to be about maturity, and you believe Barney's turn, but you're not given any reason to believe that Ted has really matured in any way.
None of this really bothers me, though. I enjoyed the show and I didn't mind the ending all that much. I think everything from the very beginning pointed towards Ted ending up with Robin. I just don't think the show runners expected people to start rooting against that possibility. They also probably thought everyone would find his romantic gestures really endearing instead of annoying. Those two crucial mis-steps, I think, are the reason why they thought they were writing a real crowd-pleaser, but ended up doing just the opposite. That's my opinion, at least.
Also, loved the hulk voice.