If you are a novice lifter, almost any stimulus will give you good results. It's why the before/afters for a lot of products are ridiculous, or why people get deluded into thinking what they were doing as a magic potion. Lifting a 6 pack will improve bicep and general arm strength in many.
Almost any time new stimulus is introduced, you will reap nice results, especially if you are a novice lifter (note: you can be great at a sport, in nice shape etc, and still be a novice when it comes to lifting and a linear progression exercise model).
Personally, I still do crunches as a finisher to some core workout routines -- eg, 50 bicycle kick crunches at the very end, as a burnout/finisher.
I do them more for mental toughness purposes, and for the variety which I enjoy.
Sit up can be good; they are often done to good effect with added resistance, since a lot of core routines are done bodyweight, being able to add rather heavy weight/resistance to a movement is good -- more variety, new stimulus.
Long answer to your question....but you should definitely do more than just ab wheel roll outs. If you just do ab wheel roll outs your core will receive a big overall improvement,
but in time, you will simply become an ab wheel specialist, and your growth/progression will stall to a greater degree than need be.
I'd pick a handful of different, quality core exercises and make circuits out of them -- you perform 3-5 exercises, rest a minute or so, and do 3 sets, eg.
It'll be easy to make the routines progressively more difficult and suited to your needs and goals, starting with something novice, and working up.
EDIT -- what you want to get out of your fitness matters most, too. if you wanna be an all-around animal, or if you have a specific sport or task in mind, etc. Most people are too weak, though, I'll say that with 100% confidence...even ppl who are very well conditioned and reasonably strong, hit major walls and fall short of their overall potential and progression bc they lack a true stength base.