I still count Ryotaro from Den-O as one of my favorite Rider characters and, on paper, he and Don aren't that different so if I like one and not the other there has to be some kind of objective failure in quality causing the disconnect.
Well, I see two problems here.
One... yeah, Takeru Satoh is a better performer than Kazuki Shimizu. Takeru Satoh is a better performer than most of the people Toei has ever hired. I think any Gokaiger regular besides Junya Ikeda would look pretty bad next to him. So, it does stand to reason that Satoh would be more enjoyable to watch.
Two, I'm not sure Ryotaro and Don really are comparable characters. Ryotaro's "zero to hero" character arc forms the backbone of every major Den-O story element besides the setting, so of course his characterization is front and center in the series. Ryotaro's problem to overcome is himself, and his own weak personality.
Don's character arc is more muted, because it's essentially the same as every other Gokaiger's, about the importance of not reacting to the world passively. Don's cowardice is cast in terms of a tendency toward trying to avoid problems, and his "bravery" is really just whatever wacky thing he does to actually engage with a challenge facing him.
I suppose it'd be reasonable to think, "Well, Don would be better if he was more like Ryotaro," but that's just not what Gokaiger opted to be about. At that point, you're really just saying, "I wish that show was a completely different show," because you're not judging it on its own terms.
The closest thing to a flaw given to him was merely a weakness shown in the one Inoue episode. Tsukasa's atittude was portrayed as a flaw, while Marvelous' was there to make him cooler.
To me, the whole thing with Marvelous's fear in the Jetman episode really came off like Inoue essentially criticizing modern Sentai for having the gall to say that characters with no flaws, who are never really challenged by anything either internal or external, can somehow be heroes. That whole episode is really about how the essence of heroism is forcing yourself to do the difficult thing, the thing you don't want to do and are afraid of doing. It's a theme that dovetailed nicely with Jetman's own themes, and in particular, with the guest star's character arc.
While I think Inoue's reading of Marvelous as a character was pretty uncharitable, I can't blame him for coming to the conclusions he does. If you compare modern Sentai to what the franchise was like in Inoue's day, that is how almost any character from the past five years would come off. I've always wondered if the reason why Gokai Gai couldn't talk to Jetman Gai was because Jetman Gai's lesson was basically irrelevant to him. Unlike every other Gokaiger, Gokai Gai faces both internal and external challenges from his first appearance. You could argue that he didn't really need the same kick in the butt that the main five, and especially Marvelous, did.
Does the writing have anything to do with that?
It more has to do with Ultraman just being seen as a fundamentally different type of show than Rider or Sentai. Ultraman is seen more as a Japanese franchise that can stand alongside foreign sci-fi TV properties like Star Trek, something to appeal to a general audience. Sentai and Rider are viewed as being more intrinsically merchandise-driven, and at best, more of a guilty pleasure or a fun thing for children of all ages.