I don't know the exact percentage of properties not in Zillow but in the MLS, but it is a very small amount. Zillow changed the game when it came to us non-Realtors(R) having access to housing price data. So much so, that some housing economist types have noted what appear to be permanent changes in months of active inventory (eg https://www.piggington.com, who analyzes San Diego's market)

I don't think it's hyperbole to say Zillow has more data on residential real estate than any other company in the US.

About that last point, Redfin has access to the regional MLSs. I imagine they have a more complete dataset.

My understanding is that Zillow relies on realtors to add their listings, so that means, maybe-it-happens, maybe-it-doesn't, entry errors, maybe-it's-updated, maybe-it's not...

Redfin also seems to have dramatically better pricing models, a better presentation, and an altogether better product.

Hard to believe they don’t have MLS access. The listings on Zillow go up essentially at the same time my realtor has gotten them through the MLS, so they must have an automated way of pulling the info…

Only realtors have access to the MLS but they can share those credentials with software products.

Source: worked on MLS ingesting software.