CannibalS
Guillotine Gorilla
In this case, in-universe and real world logistics are tied together though. When Foundation X used Medals or Switches, that wasn't only a random reuse of suits, but also directly tied to their story and nature. Rather than being random resurrected villains, we got those elements tied into the whole "merchants of death" scheme. When you bring back villains set up for a single show and just throw a bunch of random past suits there too to fill on screen numbers, they just end up as... well, random past suits.
Even by their own nature, the moment you have an army of Grongi running around, you basically just have a bunch of Grongi suits completely going against their characterization in the Kuuga tv show, where such concept certainly couldn't exist. We've already seen the Heisei Riders fighting various past Heisei villains as part of larger armies outside of the context of their series, including Grongi (like Dai-Shocker's army in the original Vs Dai-Shocker, which had mostly Heisei villains outside of the commanders and Shocker grunts), and without their context and story, they're just basically random suits. Even you're ignoring them in your post.
What elevates Foundation X for this type of role is that they were written with this kind of crossover role in mind so it avoids all of those pitfalls. Besides, I don't see why you keep throwing Shocker comparisons. The only similarity between them is basically the scale and being a recurring foe, but otherwise they were hardly the same. Foundation X wasn't even a group focused on world conquest in the first place. They couldn't take a thematic role in W's or Fourze's stories, because their scale would lead to a much more negative message than what both shows were interested in telling, but I don't see how this denies that they have a better story set up for crossover stories than any of the other individual factions.
The problem is, they simply aren't interesting villains to anyone but people who care about a comic-bookish continuity (which has no business to ever be in the general vicinity of any fiction any more).
They are essentially background noise and intrinsically have no real value to the story other than a vague thing that we are supposed to care about only because of how vague they are and superficially resemble the famous Shocker twist in Stronger but without anything that actually made Shocker work. The "merchants of death" angle doesn't work, because unlike Black Ghost, they have no real weight in the narrative other than, again, being vague and Shocker-like.
They have about as much effect in the story as the empty suits does at the end of it, the only difference being they happened to appear in the popular season while using Showa-esque aesthetics. So you might as well use Smart Brain or whatever more suitable, because at least then you most of the thematic legwork cut out for you in their respective shows and still suitably serves the idea of "Big nebulous villain of Heisei".
Or better yet, just create a new villain from the ground up that is actually unique and actually serves as a counter to Heisei Riders as a whole.
One might argue there is potential for Foundation X, but at this point I have doubts they would ever be anything actually interesting. You think Riku Sanjo is gonna ever write anything more than shallow iconography? I don't think so.