I think they’re terrific for a couple reasons.

One, the combat deck is the second half of advancement via perks. You pull out cards, add new cards, etc etc. Combat deck tuning changes the feel of your character as much as adding new action cards.

Two, you can evaluate what’s been drawn and get a feel of what kinds of draws might yet happen. There’s no removing the fumble card, so until you’ve drawn it there’s a growing chance that you will draw it. But the same thing applies to your 2x card. Meanwhile, if you’ve blown through all your -1s or 0s or whatever, you might decide to take some big gambles in the hopes of pulling some bonuses or, better still later in your advancement, a string of “effect plus draw again” cards.

EDIT: it also makes advantage and disadvantage a very powerful effect, ever moreso the deeper into your deck you draw. Very clever.