Do you even need me to turn this on you or could you do me a solid and just start explaining how any of these were any less of an ending than the final cutscene in MGS2 were nothing is explained and a bunch of exposition on meme's is thrown in my face?
I guess while MGS2's ending is definitely out there and raises tons of questions it actually makes sense as a literal series of events, or a narrative conclusion.
Raiden is slowly infiltrating the Big Shell until he gets to Arsenal Gear where he talks with Solidus, Solid, Ocelot
Etc. He then fights Solidus again on the Treasury building on Wall St before having a last talk with Solid Snake.
Nothing about the who, where or how of the ending is particularly hard to grasp, even if some of the things said are wacky or confusing. I understand the basic narrative.
Alternatively, the Phantom Pain's ending I don't understand at a basic narrative level. A mission unlocks that is a different version of the Prologue from the ACC. There is no narrative jusitification as to why that should happen, since Venom will not remember his true identity until 10 years after the game ends.
You then replay the first level, which again I stress cannot possibly be Venom himself remembering things because of the timeline, and also since it also shows stuff Venom was not present for.
It's capped off by a series of tapes called the "Truth". Now, whereas most tapes present themselves as debriefings or random conversations Venom overhears on Motherbase (except the Paz tapes I guess, but they still make sense as a manifestation of Venom's guilt) these are tapes that Venom gets from somewhere, that again he cannot actually have listened to since he doesn't have his identity revelation until 1995. There is no narrative explanation for them.
That's why people say the ending feels slapped on. Because while most other MGS games will have twists or moments of revelations at the ending, they're grounded in ways you understand how the protagonist got the info (People talking to Raiden, Eva talking to Big Boss about the Boss, Big Boss talking to Solid etc). TPP just info dumps on you without even a tangible context for why you're getting that information which is why I find it frustrating.