I agree with Larryblack and gokhan45 that the actual results tell you more than a difficulty score, because a low score means very little if the first page is full of pages with a different purpose from the one you plan to write. Bidnexa made that point well about page type. If Google is showing product pages and you write a blog post, you are not really competing for the same result, even if the words match.
What I would add is to start from the page, not the keyword. Before I decide to target a phrase, I check whether my site already has a page that is close to it and getting impressions in Search Console for related queries. If it does, I usually try to improve that page first, because a page that is already near the first page has a clearer path than a brand new page starting from nothing. Rank_Lift made a similar point about staying close to what the site already covers, and I think that matters more than finding a fresh keyword in isolation.
I also agree with Starblazer about matching the choice to the strength of the site. A smaller site can still win when the intent is narrow and the current results are thin, but it needs a page that answers the query better, not just a page that uses the phrase more often. That is why I now check the top results by hand and ask whether I can honestly make a page that deserves to replace one of them. If the answer is no, I drop the keyword, as Digital Crown Agency described, even if the tool score looked easy. Pax Digital is also right that the value of the visit matters. A phrase that brings a few people who actually need what the page offers is worth more than a phrase that brings traffic that leaves straight away.