Strong yes, but with the exception of beast of chaos, who need 2 units to be near their stone on top of being a fairly squishy army and thus will risk a lot if they try to use it, most armies should be able to work around it without too many problems.
Or well, let me put it this way; random terrain like arcane is unreliable to begin with (may not be in the right position, may not even be on the table) so being denied that should simply mean you play "as normal" and shouldn't be an excuse for why you lost. Then there are factions who have sturdy enough support heroes to eat a D3 or two and could risk it (to stick with your FEC example, the models that need to go near the throne outheal the RSE...) Most faction terrain has at least some effects with a larger range than the 3" our RSE hits anyway, reducing it to a minor inconvenience. And lastly, we can only target 1 terrain piece at a time, and rarely do people have a reason to stick multiple units near it, let alone stay near terrain indefinitly as the terrain might not be in a particularly relevant part of the battleground (e.g. far away from an objective). All of which reduces this to essentially "you might get a 3-4 D3 damage over the course of an good game" and that's without the opponent trying to deny us the terrain (or kick us out), or taking into account that we need to actually get into the damn thing first. Which isn't exactly gamebreaking. Taking into account those additional limitiations we're probably going to be lucky if we 1-2 hits with it in an average game. I wouldn't be surprised if the average damage, ignoring potential wardsaves, would be something like 0.5-1 per game...
This at least I agree with
I think I've told you something similar before (at least I've told someone on this site) but no, the fact that X & Y aren't balanced in a vacuum but within the context of their faction as a whole doesn't mean you can't compare them. And this goes doubly for things like faction terrain which are "free" for a given faction and thus part of their core. And thus should provide similar-ish value or be counterbalanced by some incredible flaw in their core design. In which case our RSE gets the worst of the deal, having limitations that no other faction terrain has & being the only terrain that can potentially be advantageous to the opponent while not really giving us much in return
unless it's used in (near) optimal circumstances. On top of that, (near) optimal circumstances require a fairly dumb opponent (e.g. nice enough to go stand next to terrain with multiple models, nice enough to not contest it) or a load of luck (win the roll-off, somehow, with our 8907439857 drop armies)
