When you want this engine powering your game?
If you expect your game to have moments where there's plenty going on, where there are three different things going on to worry about and two different awesome things the character could be doing, and you don't feel like sitting there for ten minutes making a roll for each single thing... That's the ideal moment to be using Schema, because it is better at handling that than any other system.
If you'd like to see characters needing to pick in the heat of the moment between bringing down the house or feeling sure they're going to come out in one piece... and if "Are you more okay with collateral damage on this one, or more okay with getting knocked off the roof and the bad guy getting away?" strikes you as a fun thing to happen in a roll, then you should use Schema.
If you want to characters to feel like they're patiently slogging it out when they're keeping their heads down, and throwing it all to the wind when they really care about what there is to gain, you want Schema.