Using certain debuff effects on solos is too effective. Stun attacks are the most obvious example, but Dazed, Weakened, attack penalties, and so on are all particularly strong when used on solos.
The bonus to saves that solos get isn't enough to compensate for this because many of these effects end after a round, or even saving after one round isn't enough because a party can hit with a new debuff every round for several rounds into the fight. At higher levels, characters can have many of these debuff powers (at low levels, it seems like much less of an issue).
Though, I'm also fine with all solos being immune to Stun, or downgrading Stun to something else.

Well I for one thank you for your expose. I haven't gotten around to making any of my own monsters but enjoyed reading how you broke yours down.
cheers,
Morgan, your method of development for your solo is awesome! She seems to have turned out quite nice, with quite a few interesting abilities to keep the battle fresh. I especially like her various immediate abilities. Great work!

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.