I'll try and give a balanced reply here as we do work
with one rope cutter.
I'll give you the choice and how they work
and let you research
and make up your own minds.
What happens with a prop fouling.
Debris rope/net/line is caught by a blade of the spinning prop.
It will wind
on the shaft between prop and bearing with no cutter, it will be turning with the shaft as it winds on. This debris will be very tight, so tight it can weld itself in to one solid lump of plastic if it is pp rope
There are three types of cutter available
Disc (prop protector/shaft shark) do effect water
Easy to fit.
Scissor (Spurs/gator/stripper) do effect water
quite easy to fit.
Shaver (quicKutter) need a machine shop to machine and fit spool so harder to fit.
With disc the debris will not be moving relative to the blade as the debris will also be spinning, so they only cut once debris has wrapped over the disc and then pulls tight.
With scissor they will cut debris before it winds onto the shaft as it drifts past but I believe if debris is caught by the prop and is winding on to the shaft aft of the cutter they may be overwelmed or pushed /bent when debris reaches the rotatiting blade, they can also be easily broken if something they can't cut gets between the blades such as a fish
hook or swivel. Like scissors if you try and cut something too big or too tough you break the scissors. There are many stories of mooring
lines being chopped up by scissor cutters if you drive over them but this chopping may not be chopping rope that has been caught by the rotating prop.
With shaver, the shaver blades cut debris that has been caught and is winding on the spool which sits over the shaft. Very dificult to break even when swamped with masses of net or thick rope. Used by the UK RNLI (they removed spurs cutters) the RN and fitted as standard on Christensen yachts as well as many commercial