NIW is very flexible!!!

If you have very strong background, then go to EB-1a.

If your background is not very strong but have a permanent research job, then go to EB-1b (OR).

If you do not have strong background and do not have permanent research job, then go to NIW.

But some people may be OK for EB-1a, but for safety, they go to NIW also.

Some people have weak background but get NIW approved very soon and smoothly; Some people have stronger background but did not get NIW approved smoothly (get RFE or long waiting or get denied).

NIW does not have very clear criterions like EB-1a and EB-1b.

So NIW is magic, crazy and very flexible!!! You need to "Chui (1) Niu (2)". A mess!