Something that stands out is that Space Age is more of an adventure (overcome challenges in series) where Factorio the base game was a sandbox (challenges can be handled in parallel).
In the base game, if someone doesn't like rail then they can just ignore it for an entire playthrough. Ditto nuclear/solar, most military techs, most other logistics, and so on. If someone likes yellow belts and coal power that can carry them through an entire playthrough. If you don't like Gleba/spoilage, you can't skip it because it is necessary to see any parts of the game gated behind it which is Aquilo onwards. Heaven help you if you don't like how space platforms work, because they gate everything and are quite unique compared to the rest of the game which some players must hate.
Aquilo is just so barren. The new mechanic it introduces locks down player choice, as opposed to the other planets that are rife with opportunities for exploration and creativity.
I find fulgora and gleba a bit tedious, but I appreciate that each of them bring unique new restrictions/mechanics/challenges to the game. Vulcanus is actually the least inspired, from a mechanics perspective. Mining->lava. It's a good lead-in to those wilder planets.
I agree with that, but I also think that the huge mechanical divergence of the other planets is a bad thing. Like I said, building on other planets it really doesn't feel like Factorio any more... more some new game with Factorio graphics.
I think that they are a terrific sequel/expansion to Factorio. It's still "Logistics: The Game".
Fulgora requires you to work in confined space and manage a complex situation regarding waste.
Gleba requires you to deal with stock that expires, sometimes dangerously. (Tip: Basically you need to keep refining the "dangerous" spoilable products into non-dangerous products constantly, or else burn it).
Aquilo I haven't gotten to yet.
These planets have expanded the scope of real-world logistics concepts one may think about in manufacturing, in my opinion. The expansion would have been a lot more boring if all they did was add stack inserters and turbo belts. If they are not fun, then there are mods to disable/modify recipes to not need them, while still getting all the cool new tech in Space Age.