I've been tempted to combine footpod and GPS outside since I'd like semi-accurate cadence data too, and I've noticed my Vívoactive gets cadence iffy if I have to wipe my face or drink water without the footpod.

sadly doesn't claim to be.

I can only assume it's a case of "If you're a Vívoactive 3 user, you're not serious about being an athlete and thus you do not deserve full data" which happens sometimes :/