Aim-based would only work if there were multiple sweet spots on each component, or if the sweet spot was designated independently for each component on each ship, since the different layouts and positioning of components would otherwise render a single fixed-location sweet spot a direct buff to ships where the sweet spot is easy to reach, and a nerf to ships where it's hard (or impossible) to get to.
Timing-based would only work if it's entirely handled client-side, which opens it up to being hacked/modded for unfair advantage, or if there's a relatively wide window for it, which would reward anyone who's remotely close to the right timing, make it the norm instead of a skill-based trick. Timing-based would also be difficult to balance with other players helping with repairs, unless the cooldown was displayed on-screen for any part that's visible.
On top of that, to avoid upsetting the balance of engineering, it would need to be a very small bonus - 5% seems reasonable to me, even 10% might end up feeling like too much. Or each tool could have a different "critical repair" bonus - maybe 2% on the Spanner because it's meant to be bad at repairs anyway, with 3 to 5% on the Mallet where a single hit would normally fall JUST short of a full repair and this might take it all the way, and the Wrench could take a small nerf to repair efficiency, but with a "massive" 10% critical bonus, making it a more skill-based repair tool without changing its "jack of all trades" role.
