quality of SimilarWeb and SEORush?

I didn't know whether to post this in marketing or SEO, or maybe it belongs somewhere else, but anyway...

I'm trying to understand the reliability of tools that claim to give you approximate numbers of monthly visitors to third-party sites (i.e., not a site I own, and not a site that the tool has special insight into).

I found this: https://www.owox.com/blog/articles/semrush-vs-similarweb/

I'd like to think it's saying that if the numbers are large enough (e.g., looking at websites where the numbers being shown are, say, 500k to several million visits per month), the numbers shown are probably in the right ballpark give or take 50%. For instance if it's reported that a site has 2M monthly visits, you can assume the actual number is probably between 1M and 4M. (Another question is whether a visit means a site visit or a page visit -- it wasn't immediately clear to me which the free SimilarWeb Chrome plugin that I've been using means.)

It also wasn't clear whether using the paid version of these tools is any better if all you're trying to do is estimate the number of monthly visits. On the SimilarWeb website it made it sounds like you pay for details like segmentation by country, user interests, etc. I didn't see anything about getting a more accurate total visits number.

Does anybody have experience with these tools?