A good power meter is accurate to +/- 2 %, sometimes better. If you're only measuring one leg, you don't really know the maximum error of the thing. (Leg imbalances tend to change with cadence and output.)

It'll get you close enough for training and dieting purposes, but I personally don't see the point of buying an exacting measuring device if it isn't exacting.

But I'm a software developer by day, so I tend to think in terms of "it's either correct or it's not."