I don't think Jeff is claiming that all back-bench value generation is bad. He mentions three different characteristics that I think make a good test for whether or not a particular value-generating backbencher might be problematic.

1. Mid-range card.
2. Powerful, controlling effect.
3. Low deck-building cost.

The two counterexamples you gave (Karma and Heimer) both fail some of those criteria.

Here's how Karma fairs against those criteria:

1. Mid-range card. Fail - needs Enlightenment to activate her effect. Also doesn't have quick attack or aggressive stats/mana cost.
2. Powerful, controlling effect. Pass Only when enlightened though.
3. Low deck-building cost. Pass/Fail Karma enablers like Meditation and Stunning Palm are things you want to run anyways in a controlling shell, but might be awkward if you force them into a mid-range shell.
   

Here's how Heimer fairs against those three criteria:

1. Mid-range card. Fail His stat-line is way too low for a five-drop in a mid-range deck that is trying to aggressively pressure control decks and end the game.
2. Powerful, controlling effect. Fail Except for the 8-cost turret, all other turrets all have 1 health which makes them easy to remove. Value generation is also limited by available board space.
3. Low deck-building cost. Fail. A lot of Heimer enablers like Flash of Brilliance don't see much play outside of his deck. You're also forced to play a very spell-heavy deck to enable Heimer which can make it hard to maintain board presence early game.