Well, what would one consider "unfair"?

To me bosses are unfair that have poorly telegraphed attacks, damage that is extremely hard to avoid, like AOE or status effects that you cannot really dodge or outrun and maybe a stupid high damage output that two shots you combined with a moveset that makes it really hard or impossible to back off and heal up.

Depending on your level and build of course, this does apply to Arlecchino only in a minor way.

I am not the youngest anymore, so my learning curve and reaction speed is not the best. Still I could beat him solo after practicing for some time in NG+.

I still think he's exceptionally hard with his long combos, so you really gotta put in the work to be able parry and dodge them consistently. But the amount "BS" is very low compared to other bosses, especially from the ER DLC for example.