The terrain on itself, when reading the rules on the warscroll, doesnt seem too strong, but when you actually get down to it and map it out on the table, it is very impactful. Especially when you start to pile on synergies. You can build your list and deployment around it.
If it was always in our deploymentzone, you just deploy directly into the RSE prior to the game, no need to spend movement on garrisioning it, so you can use the effects already from turn 1. Again, the effect on its own is not THAT crazy, but when you pile on things, it adds up quick. You also measure from any corner of the RSE when applying auras, so you get a much bigger coverage from whatever you put inside. So it is not a hassle and there is no real downside to it beside having a bigger coverage, which you obviously counter by screens (hence building your list around it etc.).
I think it is quite the statement my man, just because Im not cynical and want fair games. I get it that some people dont like to consider their own army as broken/OP, might even justify the power level or is simply blinded by the fact that Seraphon has been the underdog for a long time, but thats not me. I dont care if Seraphon has been terrible since the introduction of AoS, it is about the games now, not from 4 years ago.