injury does work, the effect isnt that big but it does have an effect. when i fight kakashi clone using susano sasuke i hit about 800 more with my lvl 8 injury runes than without them, not a big difference when you see 15k+ crits without them, but 1050 injury giving ~800 bonus damage to crits against kakashi is still pretty nice, sadly once player defenses are added in the number drops to ~200 bonus damage, but if ur adding ~200 damage to every crit on 4 ninjas it adds up, so if i were you i would put those injury runes in, i havent tested how well the crit damage reduction works from injury runes works though so not sure if its worth anything from that point.

i fight people with up to 100k less power than me, matchmaking doesnt work.

also when you do the math, if you crit 25% of the time, with 4 people whose standards hit 1 target and 2 of which have mysteries that hit 4 people on 2turn cds, that ends up being 4k damage before chase crits over 10 rounds, also im going to assume the defensive portion works too which would mean you take less damage so it could create an 8k damage swap if its defense reduction is as good as its damage gains.

i tested defense a couple weeks ago to see how it scales now compared to the first time i tested it, its reducing so much less damage that im trying to figure out which 8gates runes to swap them out for, injury is worth more than defense now because of the scaling(it was 11 damage reduction for a lvl 10 gold defense chakra on minatos standard attack), so if your worried about your power being too high because of nearly worthless stats then you may wish to look elsewhere cause im sure scaling it hurting you too