Definitely a buff that will hurt the game more than help it, especially in highlander. I honestly can't speak about its position in 6's and frankly I don't see why it wouldn't be banned, seeing as most competitive leagues have a variety of banned unlocks.
Pre-buff, the diamondback wasn't a bad weapon, in fact it could be an incredibly cheap one if you played your cards right. Throughout the last highlander season, I found it surely had its times. On ashville and viaduct, it was incredibly effective and catered to a play style of shutting down the enemy team's engineer and sniper, which is rather advantageous on those types of koth maps. The only real problem with the diamondback pre-buff was that it was the most situational of the revolvers, and was difficult to predict when it would really shine.
Now, this buff just dumbs down the game even more. Obtaining crits through destroying buildings in addition to backstabs is just outright ridiculous. The original point of the gun is now lost because I don't see why anyone would choose it for getting crits from destroying buildings, when they could just play the way they normally play and get bonus crits. For a gun-heavy dr spy, this gun probably wont be useful at all, but to the spies who already use their gun less then their knife and often only after a stab, its just a straight up upgrade. It requires no extra work, no thought, just boom, crit. Viperstriker explained the likely situation rather well. I just don't see how this is a sidegrade to the stock revolver anymore. You could have already been dealing the same damage with an ambassador, while actually proving you earned the damage. Why this, valve.
Feels like a similar situation to the short circuit, a weapon that was already outright awesome at times, and rewarded the user for playing smart in a balanced manner, that has now been turned into a mindless piece of crap. This is all based on weapon equip percentages mind you. The issue isn't the weapons, the problem is the majority of the community doesn't like to try weapons that don't come easy right away and have to make them adjust their play style. Sorry for ranting.