You are wise to be cautious about any new drug.
Ibrutinib was only recently approved
for chronic GVHD, after 2/3 of patients benefited from it in the reported study.
The most commonly seen side effects are annoyances like fatigue and nausea, while serious side effects are less common, but of course nobody can tell which side effects you'd experience.
The good news is that ibrutinib's side effects cease if you stop taking it, which means that if you're wiling you can try it, then change your mind if it's not helping with GVHD symptoms or if the side effects are worse than the benefit.