Impaler feels a bit too good to use as a win condition in the context of Rovers given its pretty low price tag. It's easy to justify since Fencers often force Stingers and other porc, giving a rebate on making one Impaler, and it allows to turn a small economy lead into safe mex denial from long range as a sort of reverse overdrive mechanic. I was the Rover player there and didn't feel like I displayed much skill other than clicking on the correct unit icon.

Impaler isn't as good as a missile silo as an economy tool but is much cheaper, and pretty unfair feeling on some maps that feature mex clusters or natural obstacles (Hourglass isn't an especially good map for Impaler mex denial). Other artilleries share similar property and are pretty painful to face too, but don't have quite as much safety factor as Impaler.

The Impaler vs Emissary interaction is also pretty lopsided since Emissary is a sitting duck and both units have the same cost and speed. This didn't come into play in this matchup because I didn't micro (see also: the commander incident at the end), but making one Impaler is also an excellent insurance policy against any Emissary plays. (Of course, Rover has a hard time doing much to a tank ball too since they have no real anti-heavy units, so maybe Impaler is a necessary subsidy to allow them to have a safe factory switch in the midgame)