Depends on your frame of reference. But fundamentals at top-level are hard, so by default, mastering any character and using them at top level is very difficult for that reason. But if you go beyond that and compare the EXTRA effort it takes to use some characters vs others than yes, there are definitely easier to master characters than others. No doubt.
Ayane really isn't easy to pick up. The character requires a lot of precision and patience. If you want to be solid Ayane, you really have to have a handle on the character. Most you see (outside of a select few) are pretty gimmicky. Such a style doesn't get you very far.
That's what I love about Ayane, no matter who the person is, anyone that picks up Ayane for the first time won't have the foggiest idea how to use her compared to other characters xD. Though, I wouldn't mind trying out Helena is 5U. She seemed fun and tricky in DOA4.
I usually play extremely difficult characters in other fighters haijin(nina/zaf in tekken, viola in sc5, c. viper ect) i can pick her up but i do know the level of work she needs and it's going to take me a while to absorb all her info, i understand most of her mix ups and in the lab for a bit i found a few funny things that will throw people off
basically her spin mix ups where she'll either do the low sweep or the mid jab, she has a few more options to go through from that point. she can hold the last hit of the triple sweeps and get advantage on block or throw, or from the mid do a launcher, go for a throw and or ect.
Ayane's mix ups are all about forcing you to press buttons while blocking her (and you will press a button) so she can counter hit you. In threshold, many of her stuns will force must-hold 50/50s (like KK and 6P3 for example). She puts you in threshold, you are going to guess three options every time. High/Mid P/Mid K launchers, and throws if you hold a lot.