There aren't any long term studies that I'm aware of. That was the main reason my Endo gave me when he told me he wouldn't prescribe it. Thankfully there are other ways.
Its a risk, but honestly, I couldn't live with the sides, and I can't live with my T at 144 where I was, so if it takes some years off my life I'm ok with it. I'd rather have a high quality life that is a little shorter than stick around to be miserable. I've got plenty of life insurance, my kids will be fine, much better off with happy me than hypogonadal or sexually dysfunctional me yelling at them all the time.