Our Fading Suns game has meta-currency points based on landing critical success, but failures give points to the GM. The GM had built up a decent amount of points. We got into a significant combat (not necessarily in difficulty, just the ending of a particular thread we were working on). During the combat the GM burned through most of them trying to turn failed rolls into hits. It wasnt terribly exciting use of MC, IMO.
When I GM with MC I tend to look for interesting narrative hurdles to add to situations as opposed to rerolls or +X thumb on the scale of checks. From a player perspective, rerolls and weighted adjustments feel good because you can emphasize your interests. From the GM side, it always seems entirely mechanical and kind of a let down. (Also, it seems a little adversarial as mentioned earlier in the thread.)
As an example, lets say the PCs are in combat with a spell caster in a library. Spell caster uses burning hands, but misses the PC. GM could spend a MC token to reroll the attack, or as I prefer, they spend the MC and start a bookcase on fire. Now, there is an entirely new dimension to the combat. Maybe the PCs care about this library and need to extinguish the flames while fighting the caster. Or, maybe they dont care, but the room is now filling with smoke, making it more challenging and dangerous to stay there.
At any rate, I know you were asking about differentiating MC, and I think the mechanical vs narrative or both are considerations on both sides of the screen as you asked here. Some folks like symmetrical application, but it appears im not one of them.