Since it was unbanned, Bonk has received a massive rework. Based on damage taken, a player will be slowed to snail speed when it fades. You can't block point when it's active, and using it to get behind results in you getting twoshot because you move at heavy speed or just being ignored because the speed reduction makes it useless. When they ran unlock testing in Invite pugs, the only impact Bonk even had was taking a bit of spam away before a roamer could sac.
In short, Bonk no longer allows you to go behind without getting punished and encourages teamplay, but the people who weren't involved with testing made enough of a fuss that we all forgot about it. Additionally, there still are unlocks that require testing after their reworks.
For example, the main thing that broke the Quick-Fix was its buildrate, but it now builds slower than Kritz. It no longer gets the max buildrate without damage being taken, as Valve patched that out. Its extra healing and mobility could still break the game and would require testing, and honestly if it's that broken it wouldn't take more than one game to figure that out. (For the record, I expect it would break 6s, but it should still be tested.)
With the Cow Manger, the primary complaint is the infinite ammo. The theory is that it slows down the game by giving more ammo to your demo, yet the people who ran it were roamers that usually got their ammo on the flank. I'd be curious to see a comparison of how long stalemates lasted with the Mangler versus how long they lasted after it. Honestly the only thing that's annoying about this weapon is the chargeshot when your teammate doesn't call it out. The fact that it can oneshot a light class is pretty brutal, but it's sort of risky because if you do it at the wrong time you just instantly die. I remember at one point Rekuso unbound the charge shot.
From my understanding, the Base Jumper was banned because being able to redeploy it made a soldier really hard to shoot, but you can't redeploy it. It might break the game in terms of allowing a soldier to float above any chokepoint, but that's a lot to give up Gunboats for. This is something that could again easily be tested in a few pugs/scrims.
The Cleaver is banned for increasing spam, but I suspect it might just be in a similar spot to the Wrap Assassin where that spam isn't really that impactful. This is something that, again, requires testing.
Pretty Boy's Pocket Pistol is a bit controversial. The health regen was nerfed significantly, but it still boasts higher DPS than stock. The main reason people want it gone is that it's a direct upgrade, but Gunboats, any melee ever, Iron Bomber, Crossbow etc. are also direct upgrades. Scattergun is still a lot more consistent than the pistol, so the impact of this item would be negligible with the rework.
A lot of items in this game genuinely need to be banned (Wrangler, Vaccinator, Mad Milk, etc.), but I still think that unlocks should be banned on the basis of breaking the game.