A one percent chance of a crit, with the right rune upgrades, works out to 104 blocks worth of materials for every 100 destroyed. A ten percent chance, which is easy to get, works out to 110 blocks for every 100 destroyed. When you're destroying thousands of blocks, it adds up to extra material fast.
As for the Fracture upgrade - if I'm looking at it right, it does that percentage of damage to the surrounding blocks... which means upgrading block damage alongside Fracture will stack the amount of damage you do to peripheral blocks. Eventually you get to the point where weaker surface blocks are breaking along with your actual hit location, vastly improving your destruction coverage - and thus your overall speed in demolishing the Cube.
Like most incremental games, it's a matter of economies of scale - each individual item seems paltry in isolation, but they all synergize together to produce effects greater than the sum of their parts. (Besides, any tabletop RPG player will tell you that a ten percent crit chance works out to a 19 or 20 on a d20 (the critical threat range on a standard longsword in D&D), which happens all the bloomin' time.)
After beating the demo and testing out the Fracture upgrade. It does not what I was expecting. Meaning if a block has 20 hp and you hit for 300 on it's side. With a fracture lvl at 20% (which cost a LOT btw), it does 20% of 20 to the blocks besides, rather than the 300 of damage.
Meaning Fracture damage is based on the damage dealt to a block with NO overflow damage. Making it pretty much useless since oneshoting blocks is really easy and there is no gain of AOE damage as Fracture suggest it.
Also the Click Speed upgrade is rather useless, it cost a LOT to bring it to 7 CPS when I already click at 7 CPS manually with my 'slow' method. I'm at 12 CPS with my more consistant clicking methods. I understand this upgrade is a QOL but I would expect to go over a normal CPS with higher upgrade. This feels to encourage an Auto Clicker.
Worker upgrades are rather useless too when you compare with the Click upgrades from the Runes (money per Crits and AOE), making worker 'meta' rather underwhelming. Also the Runes upgrade for workers (worker cap) is rather useless, since the cost of workers is insanely high (at least in the demo).
Tdlr: good game, but requires balancing
I'll wholly agree on the click speed upgrades. Using an autoclicker makes virtually everything else irrelevant, especially when paired with the rune upgrade that spreads out your click effects to more blocks. The base hold-click speed is in a good place, I think, but the "manual" clicking throttle probably needs to be brought down to about half of what it was.