Modern SSKs are far better than WW2-vintage Balao or Tench class (or earlier) boats, but they still need to recharge batteries, and they still need to refuel vastly more often than an SSN or SSBN. SSKs are probably OK if your area of concern is just the Baltic, Black, or, maybe, North and Mediterranean Seas. But if your area of concern is the Atlantic Ocean or all the world's oceans, SSKs aren't up to snuff.