Personally I see nothing bugged with the whole SOI concept other than that they spawn way to early and the whole fire elemental thing.
If someone has the skill, luck and idea to go out and dominate a bunch of SOI without reloading failed captures than I can not see why this is not a valid strategy.
And if he keeps reloading he is a cheater any way so the point is mute.
For the Horde can grab roughly half of them without much risk at all. The only real decision is when to use it, ad more Sons spawn later. Thankfully the Clan AI generally uses it before Archeron spawns, so that's somewhat irrelevant in SP games.
For using the Domination spell, it's much easier than you seem to believe. If Archeron is on the coast, as he often is, shipping a unit with Mind III off on a galley renders him immune to counterattack. If not, a sufficiently large stack guarding him will prevent the fire elementals from even attacking, though this takes more buildup. The point is, it is not difficult to keep a caster, let alone any unit, safe from the fire elementals, since they don't attack large stacks despite being summoned units.
For the actual luck of succeeding in dominations as you say, I have not found that to be a problem either. Unless I get extremely unlucky, I can usually dominate 5-6 Sons of the Inferno before I lose Mind III. And since I often use Hemah or Gibbon to dominate them, I have enough spare EXP where I can take Mind III again the next turn. The lower level Sons I capture can often can take Mind III as well, leading to more, faster dominations. I have not yet failed to walk away from Archeron with 10-15 Sons and his horde after I get my first archmage.
Both Gibbon, Hemah and I guess Govannon if you're Amurites, are all available earlier than standard archmages, and they all have enough EXP to take Mind III multiple times. If you see nothing broken about turning a single, early hero archmage into a stack of 10-15 archmage-level casters(Which have a much greater strength, and are thus much harder to kill by assassins), with no unit limit(as archeron keeps spawning more), then I really don't understand what would be. As I understand it, the Sons were designed to make Archeron harder to kill early on; not to function as a roadblock for those who lack the means to defeat the Sons, and not to provide a reward more powerful than his treasure for those few civilizations or religons that have early domination at their disposal.