Right, vehicle speed is a major data imput to the ECM and it needs to be correct. Alot of design is put into it to insure it is correct.
A vehicle will not “rev” the drive wheels in Cruise Mode. The computer will not allow this.
The ECM gets its speed data from wheel censors (ABS) in addition to tranny yoke sensors and constantly compares the two. It correlates every wheel. It knows when one is spinning faster than another.
No manufacturer that has liability lawyers and legal departments is going to design a CC system that is unsafe to use in the rain and the sell it to Kathy Soccermom. They just would not do it. The “safety” for little Johhny and Suzy is too big a liability.... and a selling point.
That being said....
No don’t use it. It is not unsafe as far as the vehicle is concerned. The vehicle will be fine. The vehicle knows what to do.
But if its bad rain, seems to me you would want to have total control. CC is a luxury that kinda lets you relax. You would not want to relax during a storm. Wait till it clears, then relax a bit.