SeM Rush - keyword difficulty numbers

Just signed up to SEMRUSH through a group buy. In terms of the keyword difficulty what is regarded as easy, medium, difficult etc. Can someone give me ranges please. e.g.

Is it 0 - 40, 50, 60 is easy ??

Also, assuming that if a keyword is showing as zero difficulty this is simply a mistake or there aren't enough searches?

is there a balance between searches and difficulty which makes it worth going after? For example k/w difficulty divided by searches should be above/below a certain number?