The problem is when you are shit at parry the game gets really hard and frustrating. In dark souls you could find the playstyle that matched your ability. In Sekiro your just fucked. And if people are legit frustrated then yes, it's somewhat poor design.
You can't please everyone and From really took a cool risk with this one. The truth is that the other Souls games had really easy ways of progressing compared to this. If someone has played all the Souls games and can't beat bosses in this one, I'd guess it's possible that they have always summoned players for boss fights or looked up the cheesiest methods. All of that is fine to do, obviously, but this game requires the player to actually play the game on their own and that's not bad design even if it turns some people off.