My daughter has been on Imuran, the sister drug of 6MP. It has actually been the only immunosuppressant she had trouble with - she had infection after infection. It also comes with a higher risk of non-melanoma skin cancer, Lymphoma and a very rare type of Lymphoma (hepatosplenic T cell lymphoma) that is very serious and often fatal. It affected adolescents and young adults (more males than females) who had been on 6MP.
My daughter has also been on MTX. She really struggled with the side effects as a teen, though it did work for her. We tried several things to deal with the side effects - switching to the shot from pills (caused mouth sores), doubling folic acid dose, Zofran, lowering her MTX dose, Leucovorin (folinic acid) etc. But none were enough. She eventually begged her doctors to stop it and they did. But now she is re-trying MTX as a young adult and has no side effects at all!!
She found the MTX injection much easier to tolerate than the pills.