The cruise gets it's signal from the speedo cable, but there are several things which can prevent it being set. I've got a bypass on my clutch start switch but it's a completely different switch. (Start switch is at full pedal depression, cruise switch is at the top of the pedal travel.) That way hitting the clutch at all turns off cruise, even if it's not far enough to allow the starter to engage.
I know that the cruise ECU will refuse to set at less than 40km/h and will lose its resume setting below that speed as well. It will also refuse to set if the e-brake light is on, whether that's from the e-brake being set, or from low brake fluid in the master. There's a separate wire from the cluster to the cruise ECU for this.
Also, the cruise will disengage if it can't maintain speed. I'm not absolutely certain if this is triggered by the speed sensor, or by the throttle position. While my car was still NA, I've had it disengage going uphill on "resume" while still accelerating, so it acts like it may be based on throttle position. I've never had the cruise disengage on me after I swapped in the turbo engine, even when it can't maintain speed on long uphill segments (Because going into boost backs off the throttle) which seems to corroborate the idea.
