I was a roadie until getting hit and hurt. I still have a road bike and have nervously ventured out on the road again a little but have been doing more gravel grinding, CX and single track.
As for which is safer, anything can happen anywhere and at anytime so live your life accordingly. Weigh the risk and consider who you are leaving behind and what your death or permanent injury will do to their lives. I am not all doom and gloom about it but it needs some consideration. If I die, I certainly won't know or care but I have a family and it will affect them.
While I would like to again ride carefree and blissfully on the road, I am having a hard time putting myself out there.
I am hoping that in time my love for the road will live again. I have invested in more safety gear and am getting involved in cycling advocacy in my area.
Like mentioned above, the MTB route does not so much place your life in the hands of every vehicle driver in your vicinity as it does on the road. Every car that passes you could be the last one to do so even if you are doing everything right.