I pulled 600 keywords that three popular tools all scored as low difficulty — under 20 on their respective scales — and checked what was actually ranking.
What I found in the top 10
At least one site with 500k+ monthly traffic: 71% of keywords
A forum or Reddit thread in the top 5: 44%
A result older than four years with no updates: 38%
An actual small site under 10k traffic in the top 3: 9%
So nine percent of "low competition" keywords had a small site visibly winning. That is the number that matters and no tool reports it.
Why the scores are wrong
Every difficulty score is a function of backlinks to the ranking pages. That made sense in 2015. It does not describe a SERP where a 200-authority brand page with eleven links outranks a perfect 60-link niche page because of everything else the domain has earned.
What I do instead
I open the SERP. That is it. I look for at least two results that are weaker than what I can make — a thin page, a forum thread, something stale, an irrelevant result the engine clearly settled for. If I cannot find two, I skip the keyword no matter what the tool says.
It is slower and it does not scale into a spreadsheet of 10,000 rows, which is exactly why it still works.
Link builder. If it ranks, it is whitehat enough for the client.