It depends on the pen and the ink. Taking a regular dye ink that has dried up in a pen you can disassemble the section of, it won't ruin the pen because the ink will dissolve when the pen parts are submerged in water. A more complicated pen or ink will make a pen harder to get back in shape because the you cannot get the ink out or the ink was chemically distorted the innards of a pen.
I suppose you'll have to define complicated... Are you thinking of older pens? How about just any modern pens?
or what about an older 1950's piston filler?