The good points of the forgotten art of knee paddling ,, there are bigger groups of muscles engaged, so it can be more efficient, pop ups are easier but a little different.
Why the fear of being seen knee paddling? Kookitis ?
You have a different eight position on he board due to your body weight bent more condensed! So although your nose height above the water will be the same, your position will not.
You need a heavy deck glass job or you will quickly get knee dents, you will develop surf bumps, surf knot board lumps on both your knees and feet! If you do a crooked toe type paddle, you have chance at weird osteoarthritis toe damage.
Get really good at high speed knee paddling try this paddle out really fast towards a wave about to break use the speed to stand up and do a top turn back down the wave, thereby catching the wave without paddling for it!

I've taken up troll hunting just for fun, instead of a rifle I'll just use a pun! 冲浪爷爷