I'm actually not sure if this is just an sc2 issue, but some of the units just have completely broken clickboxes. I've had this happen where odin can't be transfused when my mouse is directly over it, but that might've just been because there was a prism nearby with a broken hitbox since I can't reproduce it.

Part of the issue seems to be the idle animation changes the hitbox, and the units having holes in their model (sentry screenshot seems like mouse is on a tiny hole on the sentry). Since it's so finicky already it makes it extremely inconsistent to target these units.

attached screenshots where my mouse is clearly over the unit, yet it shows I'm hovering something else.

suggestions for quick slight fix: use the regular sentry model and maybe remove idle animations