Yes, it is unbalancing. Consider the epic Martial Power feat "Whirlwind Sneak Attack", which allows you to deal Sneak Attack to all targets of a power that you have combat advantage over. Applied to the Level 23 power Steel Entrapment, and with Backstabber it becomes Close Blast 5, 3[W] + 5d8 + Dex + immobilize target. Compared to say, the Level 27 Wizard Encounter Black Fire that is Close Blast 5 for 6d6 damage, this already seems a touch unbalanced, but wait! For just a few levels more we'll throw in a heaping load of cheese!
Mix with Epic Resurgence (regain encounter on critical) and Daggermaster (crit on 18-20), and Wraithblade (extra Sneak use on crit) for a sprinkling of cheese. Even better, coat the whole thing with cheese by mixing in Deadly Trickster's Trickster Control feature at level 24 (if you roll 18-20 on a power, the power is not expended), or use the Eternal Seeker's Seeking Destiny to get Trickster Control and then use the other features to really make things crazy.