clorgPersian Persuader: This weapon is amazing. All demos will switch to it while holding last because there's no reason not to, you get a free health pickup+the resupply gives you your ammo back. Also rewards demos being roamer crazy and diving for picks without dying cause of the amount of health you can pick back up from dropped weapons etc.
The reason not to is that you have to go back to spawn on the push-out. You also only get 20(?) health from a weapon drop. It's not much.
Eureka Effect: Engineer puts himself out of position to set up a dumb sentry/teleporter spot then goes back to spawn by pressing right-click. Yeah, no thanks.
Then you destroy the sentry/teleporter. That, or you kill the engineer since the taunt takes time. It'd be unused except for granary (and maybe metalworks but there are multiple viable spots for a gun there so probably not), and even then it wouldn't be that much of an upgrade.
Diamondback: Already discussed, getting kills and turning them into 102 crits is stupid strong.
It's strong, sure, but you need to be literally point-blank to 3-shot a med without crits, and you need to get a kill on spy and not instantly die for the ability to be relevant at all.
Enforcer: Everyone would be using this and run around with spy kritz because it's amazing.
It's not a kritz thing, it's more of a "this gun just gives you bonus damage for no reason" thing. Also, it's so stupid it's banned in HL, so take that how you will.
Spycicle: Straight upgrade and silent back kill. Why?
Comms are a thing. I'd actually say it's worse because most of the time you'd run a spy is when pushing last, which means there's a decent chance of them having a pyro. If they do have one, I'd rather have stock knife so I don't randomly lose it when going for a pick. If not, the abilities are literally irrelevant.
Powerjack: Promotes annoying pyros to be even more annoying! Get to mid as medic and a pyro is air blasting you into a wall while the enemy demo stickies you and roamer jumps you. Who wouldn't run this weapon anyways, for axtinguisher or what? Terrible idea.
If you're running pyro to mid, you're gonna get destroyed by scouts. Not a whole lot to say here. How is this hypothetical pyro teleporting in front of the medic?