uhm, I wouldn't agree with that all that much, not for this reason at least.
suppose you have 1000hp,50 % defence and 100% reflect damage
suppose I have 10000hp, deal 2000hp each attack and blablabla
things being like that, you would still die in one hit from me.
reflect damage has value only so long that you also have the defense and hp to go with it, it all depends on the order of magnitudes.
if he's attacked by someone VASTLY stronger than MC, who could 1hit ko MC with, say, using 10% of their power, that said attacker would get rebound damage equal to 10% of their power would be a "shrugs, drinks an healing potion" from the POV of the attacker.
And by standard cultivation logic, "vastly" stronger than MC will be basically everyone ever with an higher cultivation rank, it's ever the cliche that the gulf from a rank to the next is described as "enormous", to the point that "Rank X+1 cultivation level can effortlessly kill thousands rank X cultivators with barely any effort".
Flat %damage reduction is the gimmick which lets this MC overcome that in this particular flavor of the usual soup and I do agree with picking it over reflect damage.
Because it's not the battle with the mooks that make or break an hero, it's the boss battle!
now let me sulk in peace for defending shadow hack, I feel dirty :(