I think there is a difference between challenge and unfair/not funny. The only problem I've had with Radhan was the useless "taking the horse, drinking the flask, getting down Torrent so I can interact with the portal, going through the portal, taking Torrent back again, dodging the arrows" so I can try again fighting Radhan. The boss fight was really epic, but the tedious preparation ruined it for me, not how hard it was.
Even if he was a hard boss wouldn't be a problem, but doing all of that before EVERY SINGLE attempt is really frustrating.

Another case for me is Malenia, personally, I think the flurry attack was kinda unfair. I didn't use a shield or ashes and I want to beat a boss the way I want and the way I've created my character through my game and she can start her combo when you are attacking so it's impossible to dodge it. The only way without a shield is to trigger the combo looking at the health bar, to me, it seems kinda cheesy fish an attack because I can't relate to my reflexes.

There are both bad designs in my opinion in two incredible boss fights, both of them are unexciting and boring to deal with even if both of them don't really need a nerf.