It analyzes the audio files (takes a bit of time), not only the metadata. The BPM is sometimes off, especially for the loops not in 4/4. They key is sometimes funky too but I'd say it's around 80% accurate. When having thousands of samples, it's a great starting point.
Nothing is ever free per se, as with FB if the product is free then you are whats for sale (ie your data and/or your role as a potential customer for what they do sell)
its also worth noting a sound library is for life, not short term. For me, SoundMiner has paid for itself many, many times over... I appreciate the upfront cost is hefty, but managing a huge database of sounds so you can work efficiently in a professional sense is worth a lot. I dont remember exactly when I first got SoundMiner but it would be over a decade ago, so the impact on workflow (& the tax write off) means it cost me less than nothing.