so . . . To keep this in perspective, we're talking about a ganged voltage/current meter with a fancy calculator for wattage, an integrator for mAh, and a few resistors tossed on to balance-drain a pack, in a hansom aluminum case . . . all for $20.
Accurate? To the stated precision? Maybe. Probably not. Close enough, though?
Drift? There's a chance of it, but it's all solid-state, so I'd suspect more of a chance of thermal drift if the device lacks compensation than from any other source . . . but by how much? probably not much at all.
At this price, professional gear with quality and conformance certifications is unrealistic. If you need the right number to a fine measurement, every time, then this device is not for you. If you're trying to build up a lab rig to precisely measure for a commercial or research purpose, this is not a place to cheap out.
If you're building a test-bench to double check your plane's power system against specs, or measure current draw for sizing, or to gather real-world motor measurements for yourself or other hobbyists, It should be just fine.
For "hobby" use, I wouldn't worry too much about those last few digits. Yes they may change, but if you measure a 32.1A one time and 32.5A another, will this change the sizing for your ESC? C-rating on your batteries? Whether Prop brand/size "A or B" is better suited to your needs? Whether your motor is Ok/border-line/in-danger? The difference is down in the noise -- The precision and accuracy may not match the significant figures given, but for these purposes, you need the rough number, not the precise number.