I never really hear experienced NIN players at least say that NIN is hard, I think it's usually people who have just '' tried it out '' who think that the Mudras are hard or people who haven't played it and just look at them and think that it's hard to memorize and perform.
Mudras did use to be harder to use too and very prone to screwing up due to server lag they've changed VERY drastically over the years.
They used to be super fast and Raiton wasn't even used most of the time in single-target because of how fast they were and how easily latency would screw it up if I remember correctly it was only a small gain in perfect scenarios with an absolutely perfect connection.
And then they had a lot of utility that required some actual knowledge of the fights on a deeper level than other DPS.

Nowadays tho I don't think that NIN is harder than others and I really don't see people claiming that it is either except for some exceptions who probably don't even know how they play currently.