Funny thing is, I didn't like Kiki's ending. I thought the whole dirigible climax was wholly out of character with the rest fo the film. I mean, yeah, it was exciting, but it felt like a dramatic thing just put there to end the film "big". (As an aside, I still consider Kiki a top 5 Miyazaki) As for Porco, it has its own flaws but I don't want to make this post longer than it is.
As I noted earlier though, I thought Ponyo's last act was weak also so I agree with you there. Where I disagree is the notion that Ponyo's ending was weaker than the endings in Miyazaki's other films. In fact, I don't think Miyazaki writes good last acts in general, so when I look at his films, I look at them as a whole, and not necessarily from a story based standpoint (because I think there are many anime that do plot-driven stories better). In comparison to some (in my opinion) lesser Miyazaki films, I just think if I wanted to watch something like Laputa, I'd rather be watching Future Boy Conan or Nadia. If I wanted to watch something like Princess Mononoke, I'd rather be watching, heck, Nausicaa. If I wanted to watch something like Howl's, I'd rather be watching Princess Tutu, and etc. Each of the above Miyazaki films have, in my opinion, a superior comparative alternative. But with Ponyo, there isn't a comparison because it's structured differently from most anime. It has a threadbare story, but its observation of normal children's behaviour is second only to one other anime (Totoro). It doesn't use the hyper-detailed "realist" backgrounRAB most anime use. It doesn't fall back on loli stereotypes in order to look cute (Ponyo isn't sexualized like most kid anime girls) and Ponyo's entire thrust is in service of one focus, which was Ponyo and Sosuke's bond. Even when they're playing around and doing nothing to move the plot along, it's still consistent with the film's focus.
Howl's was hampered by a largely unnecessary war subplot that takes up a good chunk of the movie. Diana Wynne Jones (the writer of the original novel) hated this addition, and while I don't "hate" it, I disagree with the decision to insert it in. Instead of paring the novel down and distilling it to something more focused, he added in a totally superfluous element that comes off as preachy and clunky. Even Miyazaki's reasoning for putting it in (to make the film "relevant" to today's politics) betrays a somewhat questionable creative decision on his part. If he took that out, and focused solely on Sophie and Howl's relationship, the film would have been just right. But every time Sophie and Howl seemed to get close to developing their relationsip, something happened to interrupt it, or the annoying thing known as "plot" started kicking in. So instead, it's a noble failure in my eyes, but a failure nonetheless.
EDIT add: After some brief pondering, what ultimately makes Ponyo work for me despite any writing flaws is that evokes a sense of emotional nostalgia. It's not nostalgia in the sense of "Oh yeah, I remeraber the old days of collecting Transformers toys" but rather it makes me remeraber the various things I was thinking about as a kid. Playing along the local creek picking at and examining crayfish, playing with my frienRAB in the summer and running to the popsicle truck when I heard the overbearing melody come by, and being absolutely terrified when I was in a mall, got lost for a bit and couldn't find my mother. Now, Ponyo doesn't show *exactly* these specific moments in my life, but many scenes in the film evoke these sort of long-dormant memories. Now it's true, a kid's show like Arthur might portray the same things in certain episodes, but not with the same level of skill--it's still portrayed from the perspective of an amused adult author. Where Ponyo goes right is that it shows these things like it ABSOLUTELY MATTERS and emotionally is EXACTLY the way you remeraber it. Now, I have no idea if the stuff portrayed in Ponyo resonates with today's generation of children since I feel like the lifestyle of children of the past 15-20 years is far different than those of someone born earlier than, say, 1990.
But yeah, as you said, it's all subjective and that's just my take.