I still can't see why would it be balancing nightmare. [Omnivamp]'s definitely easier to balance than ordinary lifesteal
It's not.
On glass-cannons Drain offers an interesting risk/reward dynamic.
On tanky champions it offers tank'n'spank snooze-fests, or tons of reward for no risk.
Tanks and juggernauts are the true hybrid damage champions, and with 50% resistances turning 10 HP restored into effectively 20 more HP: tanks and especially juggernauts could break the game with omnivamp. Especially considering juggernauts tend to do carry-level damage as-is.
They just suck at drain-tanking ATM because there damage is split between auto-attacks, abilities, physical, and magical damage with %HP and True damage occasionally thrown in the mix. That mixture has always made effects like life-steal and spell-vamp 50% as effective on Juggernauts as glass-cannons.
Life steal is easier to balance than Omnivamp as it's more restrictive and offers more interesting risk-reward dynamics.
All that said: 2% Omnivamp in a pretty small amount.
It's just not something you'd ever want to see on any item tanks and bulky fighters would consider, as that's where the balancing nightmare would be.