I think what makes 5 difficult is related to what makes 7 easy.
I think 5 is counter-intuitive that there are a few longer routes with close to no benefit (woop de doo, you picked up ~12 blue spheres) that set you back a ton - it's better to just take the shorter route and get those spheres further down the track while also being closer to the UFO (which, thankfully or horrifically depending on your POV, doesn't have rubberband AI).
Shortening your locus in 5, avoiding grass, avoiding bombs, and picking up as many blue spheres on that route makes it consistent.
On the flipside, Stage 7 has a shit ton of corners you can cut and it's completely possible to easily nab the UFO at Mach 2.
edit: The relationship being the way the pickups are positioned in 5 (longer routes) puts you at a disadvantage, while the way pickups are placed in 7 (especially the bubbles) teach you to cut corners.