Well, the main issue stems from pure success being unlikely to attain because most rolls will not be for 'sure things'. Your combined chance to fail a roll outright or succeed at a cost is far far higher then succeeding a check, so when I played it, we entered a failure cascade pretty fast whenever we had to roll to do something.

Like, in any situation, you only succeed without issue if you roll a 6 in your die pool. Typically, that pool is between 2 and 4 dice, so your chance to roll a 6 is, at lest, okay. And if you don't get that 6, you either have to escalate the situation or suffer complications, or you just fail, if you only have 1-3s on your dice. It's just...I dunno.

edit wasn't done typing: If you had more dice, this system would probably work, but each class is only good at very few things, so as soon as you are out of your characters comfort zone, you are in such high risk of failure that it's just not very fun. When I played it (one time), it felt like a comedy of errors and not like a bunch of hardened criminals doing a heist.