I understand that they can't make armies on their own and that's probably a good thing, but maybe the -2 is a bit too much, at least offensively. Maybe make them cost more hammers and be a little weaker, but in a world where two points is the difference between an entire techline, it basically means you're better off suiciding legates at improbably low odds to get reborn in any war not in your borders, because they're cheaper and have THE SAME ODDS. That's a problem to me. A big one.
A nice thing about the border-based strength adjustment is that it does fit with other Scion effects, it's a lot more flavorful than just making a unit more expensive. So I wouldn't want to abandon it completely. But nerfing the nerf and and adding an increased costs sounds pretty good, or making them a nat. unit.
I would suggest that maybe we make it a -1 nerf. That I could live with, seeing as they do allow for Reborn. Also, I would ask that either a new, stronger spell on the level of the other teirII Priests.
OR (and I'd prefer this option myself) make Weird Wrack a stronger spell. I'd suggest it be slightly weaker that other teirII divine spells, say 10% to 15% (possible a MAX of 20%) weaker simply cuz this would allow for more balance and force the player to be more creative with the use of it. It would also keep the unit from being overpowered, as it already does one significant thing by allowing Reborn to be taken.
Better solution than nerfing Doomsayers? Nerf Doomsayer HORDES. Make them a 4-6-8 (unsure of good number, maybe base on map size?) limited unit. Then you can't compose a stack of doom with them, but they can be strong unique units you want to protect and nurture rather than spam in the hopes of getting Reborn.
The only problem I see with them being national units is that the national unit limit seems rather arbitrary. (Not that I haven't used it before...) OTOH it'd address the "horde" concern even more certainly than increased cost and could be used as an excuse to make the units even more powerful. (!)
I personally would be against limiting the hordes. To me, this seems like a cheap way out. Basically, it would do this. (Go on a lil logic jaunt with me please.) We would have unnerfed Doomsayers/Doombringers. Then we limit the size of the armies that could accompany them. Let's assume that most players will bring along a Doomsayer(s)/Doombringer(s) for every army they send out; cuz most of them will, especially those players familiar with how useful teirII and teirIII Priest units can be. Ok, this done two things. The 1st, obviously, unnerfs the Doomsayers/Doombringers. (I know, it's a "Duh!" moment. Bear with me.)
The 2nd will reduce the overall cost of the Scion armies, at least potentially. (Meaning we're assuming that most players would send out less units cuz the size of the armies are limited to how many units can accompany their Priest units.) We all know that the promotion "Heavy Formation" costs -1 gold per turn in support costs. So, if the size of the armies is smaller, the support cost would be lower.
Your 1st thought is prolly "THAT'S GREAT!" My 1st thought is "Wait, wait a sec, this tips the balance
too much." (Read my sig, that comment makes perfect sense in context of my sig.) The reason that -1 gold per turn in support costs is there is prolly something along the lines of reflecting that there really aren't a ton of Scions around to support overly Huge armies. I mean Scions don't have sex, have kids, and gain citizens in the normal way. So, obviously, their average city size is smaller than most civs. (The only civ with an average city size on the same level is the Legion of D'Tesh, another undead civ, albeit a more violent one, that's featured in Orbus and was originally an addon civ for original FfH back in .30 days.) Therefore, lorewise and logicwise, I think the "Heavy Formation" promotion on many of the Scions melee line already does a good job of keeping the Scion army size from being overly Huge armies, while allowing the player to spam units like Centeni or Horsemen for fodder.
So, please, leave that part alone. I maintain the army size doesn't need to be limited to a number X
just so we can have a stronger teirII and teirIII Priest unit. No thanks, I like it how it is; it works imho. ("If it ain't broke, don't fix it.")
Also, power for the sake of power is nice. Except in this case, it takes power from melee units (who, let's be honest, are mainly powerful cuz of their numbers ["Spam warriors" & "The more bashers you have the better"]) and gives more power to the the Priest units (who, again let's be honest, it's not useful, not worth the hammers in the case of Doomsayers, or not possible, in the case of Doomgivers, to have tons of). I don't see how that helps, to take one useful thing from one unit to offset that another unit is buffed.