All radars are 99-100% accurate.
The question is, where along the path of the pitched ball from release to the plate, does that radar gun first get it's reading.
Some radar like the Pocket Radar, you may get one reading at release and the next reading 10 ft out of the hand. And you have no way of knowing that it happened. (So to me, it's useless).
Some like the more expensive Ball Coach version runs continuously, so you can be 'sure' that it picked up at release.
Guns that display two readings are best - release and plate speed. Then, if you get the normal 10 mph difference you know that particular reading was correct.
Of course, some recent Stalker versions are excellent and are used by scouts, so you would have the same gun they use which is always good.