It really should be put in the context of someone's life. Did they spend a year or two travelling? Did they spend it working on personally productive projects? Or did they do nothing but play video games or do drugs?

These are all very different extended gaps. The first two should be fine - if someone's skills have atrophied than that's another issue (but nothing a good interview or probation period won't pick up on if it's actually even a problem). The second one might be fine or a problem - and I can see why employers would be wary.