You are talking about two different parts, doing two different things. Forks are thicker. and usually 1/8th or above, rear triangles if hollow can be a little bit thinner.
I am not talking about a weld ripping out, I am talking about tube bending at the weakest point. Your rear triangle has that cross bar as a stabilizer, If you go drilling into bar, it is likely to more to bend and snap. It isn't the biggest they can just become misaligned and make the rear tire more prone become warped.