Tomcat, I’m sorry, best wishes for his recovery.
Brain tumours, whether benign or malignant are never good.
They come in a myriad of different types and can present with almost any symptom you can think of.
Brain surgery is not fun, and for every miracle cure there will be a scare story. In this case, more than any other, it is not helpful to compare with other people, because so much can depend on EXACTLY where the tumour is and EXACTLY what it’s made of. A millimeter here or there can make a big difference when you’re talking about the brain.
Listen to your FIL’s doctors, they’ll know what the story is, and should be able to answer your questions much more accurately, given their knowledge of his specific condition.
Most people with benign tumours, assuming the surgery goes well, usually go on to live long lives. Some of them will never fully recover (your FIL may have some permanent paralysis for example), some will. It’s too hard to say.
My prayers are with your family for everything to go well.