I looked at the statistics in my county and roughly 50% of property owners protest, which is almost 200,000 protest per year.
The infrastructure deployed by the county to service this level of protest is fairly massive.
The success rate for the "informal" process is over 70%, meaning 70% of people get some sort of reduction. Might be $1, might be $1M.
I looked at actual examples this year where a $2M assessment was reduced by $1M. That's a massive amount of tax money.
Going to a formal protest (which requires putting a deposit down to have the case heard) the success rate is 48%.
Statewide 60% is the average overall success rate.
Draw your own conclusions from that. There is a "very healthy" 3rd party industry around this that will "protest for you" for a cut of the reduction. You can do it yourself, but I've found that it's a roll of the dice depending on what arbitration board you get. They are not lawyers, so they don't know property tax law and can choose to ignore anything that you provide.. And I mean anything, they wouldn't accept city re-zoning documentation on my property. Stated reason "GIS data says different" (GIS data lags).
They are reported to a privately owned system, which is how Realtors keep a their "lock" on brokering property and keep income flowing into the system. Want a "comp" - you need a Realtor. Want to list a property in their system, you pay a fee and/or need a Realtor.
What you see reported on many websites here is "asking price". IE - Zillow/Redfin may give a "sales estimate" but the actual selling price of the property is kept within the MLS system and is not publicly reported. You'll see "sold" but the sales price is not provided, just the asking price.
In my state it was illegal to sell this data outside of that private system. My county partnered with the underlying data services provider and did it anyway:
This isn't "conspiracy theory" - it's actual data and various counties doing illegal things.