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).