They *can* be remarkably accurate, in the right circumstance. In areas where the homes are homogenous, you’ll find that they can be remarkably accurate.
In the real world, where I live, the homes are not very homogenous, so those services are a lot less accurate. They tend to average the price per square foot of sold and for sale homes nearby, and apply that to the square footage of the subject home. If you’re in an area where there are huge multimillion dollar homes on the bay, the 900 sf 2br home that’s 50 years old an needs remodeling that’s a mile inland, you’ll see that the online service grossly overestimate the value of the older home. In most of the real world, the variations aren’t that extreme, but the factors that the online services use are imperfect, at best.