The Screwdriver, the green variant of the railcannon, a weapon contextualized as the hard hitting but cooldown having weapon in Ultrakill's small but diverse weapon cast, setting the standard pretty high for this alt in particular.
The Screwdriver itself fires a drill projectile when fully charged that deals 3 damage on impact and 7.25 more damage over the course of 7.5 seconds, dealing 10.25 damage overall, {the most single target damage a railcannon can get compared to Electric's 8 and Malicious' 8.25 max} in those 7.5 seconds that the drill sticks to an enemy {Killing an enemy before duration is up gives a one second grace period}, the blood healing radius increases allowing extra distance while getting the benefits of healing.
Alt fire like every other railcannon serves as a scope of sorts zooming in allowing for more precise shots, useless in a game like Ultrakill however this was intentional due to Hakita not wanting people to bother him about the absence of an alt fire despite the railcannon's design to only have a primary fire.
You can also punch the drill whenever it's in an enemy to add an additional 4 damage to the punched enemy and sending the drill flying in the direction punched with ZERO vertical dropoff, it can be moved from enemy to enemy which resets the drill timer allowing for the DOT and healing effects to last longer, an unique interaction this has is with Maruices, as their corpse still allows for drill punching despite any other enemy corpse not allowing this.
The drill won't produce a blood fountain on sanded or blessed enemies however they will stick on, still dealing damage to sanded enemies {not blessed ones though, blessed enemies are invincible} and still allow for drill punching helping increase lifespan.
The drill also has a few interactions with certain enemies, most notably against Mindflayers, shutting off their ability to teleport as long as the drill is on them. Other interactions include slowing down V2 and attracting the aesthetic lighting in both 5-2 and 5-4, allowing for bursts of 30 damage, albeit this is realistically only viable for Leviathan
With all the "what it does" out of the way, let's get to the main question-
WHY DOES IT SUCK AND WHY IS IT GOOD THE WAY IT IS?
For why it sucks, it's the DOT effect, in the 7.5{+} seconds that the drill is up there is a LOT more you can do with your time and most importantly, a railcannon charge. Railcoining with the Electric or core nuking with the Malicious are way more damaging uses of a charge, leaving the Screwdriver without anything to add immediate damage. The few uses it does have like the healing radius and Mindflayer teleport stunning can also be nullified by simply dealing damage, Mindflayers having an instakill tech with the Malicious & the lack of taking damage because everything dies so fast nullifies the Screwdriver's uses even more creating this weapon that has the cost of a railcannon charge, a valuable resource of the likes of coins, yet is still outclassed by so many other options.
However, I think it's good the way it is, it may be outclassed horribly yes, but typically it takes a high skill to get stuff like railcoining down in an environment like Ultrakill's. So a weapon where the main benefit is to be low-mid skill friendly is a really good addition to the weapon roster. The DOT allows for more focus on dodging things, the blood radius buff allows for healing from further distances not to mention it being active for 7.5 seconds makes it REALLY good for faster bosses where blood is hard to come by like Gabriel and the prime souls {although do mind parrying the close up attacks causes accidental drill punches} or attaching to a tanky enemy in a room that demands constant movement as a portable blood fountain. On top of that, the unique effects makes some of the toughest things easier to deal with, such as Mindflayers being unable to teleport or V2 becoming slower, allowing for more ease in besting them.
TL:DR, the Screwdriver is horrible in higher level play due to being outclassed, but is a beginner friendly weapon that helps newer players out a LOT. Which I believe to be why it's good the way it is.
Well turns out Violence made this weapon better since a drill punch deals the remaining DoT damage plus the 4 you get normally for punching it {for a total of 14.25 damage at an INSTANT}, so for small single targets like Gabriel or Minos Prime, this has potential to outshine the Electric in that regard {although larger targets like Sisyphus Prime will still prefer railcoining for max damage, altho the Screwdriver is FAR easier to do in the heat of combat}
With this buff in mind, the Screwdriver has promoted itself from "purely support" to "support + VERY HIGH single target damage" with a few situational uses in higher levels in play. However, the Screwdriver at its core is still a very beginner friendly weapon built around its support features, it just now has a less support orientated use for people who still want to use this thing as a damaging weapon without dealing with the DoT