Karve is definitely better if you are sailing in shallow areas, navigating around rocks, dealing with choke points and being far easier to turn around especially in confined areas.

Once I got the longboat I only used the karve once to reach my first dverger fort that was on a small island surrounded by other small islands so using the longboat would've been more cumbersome + I knew of at least 1 Gjall being somewhat nearby so I wanted that extra versatility of the karve in case I had to reposition for an escape if needed.