I have had many cockatiels over my many years and a good share of them were rehomes, Some from good homes and some from not so good homes.
They are small birds with big attitudes and sharp beaks.
If they came from a bad home and not socialized, abused they than can tear you up big time.
All parrots if not treated right or abused can give you a bite you will not forget soon.
I have the scars to prove it and they came from some of the smaller birds.
Big or small, they can and do bite.