My passion on this bike rack deal is with the lack of details provided by folks so say "don't do it." If you don't really know the details of what they did, you don't know if they did something obviously stupid (my guess, since they won't give details), or if they really did something most of us would deem "reasonable," but something else went wrong.
Welding on additions in almost all cases will strengthen a frame, but drilling a hole will always weaken it, to some extent. So, drilling a hole in the flange of the frame just didn't seem like a good idea to me, no matter how good or bad the frames on these things really are. That being said, it's probably not a big deal, but my method has some other advantages, as discussed below.
I attached my sewer tube (5"x5" fence post) by using a pair of bolts on either side of the frame web. A 2" fender washer inside the fence post, and 1-1/2" fender washer on top of the frame flange. Stainless steel washers, nuts and bolts. Since half of this washer is over the web, but the other half will hang out in space, I cut another washer in half and captured both halves between the 1-1/2" washer and the fence post, basically making up the thickness of the web. So I have a total of 8 bolts - 4 bolts on each side of the trailer - two on each side of the frame flange. See the attached drawing of one side (it's not perfect, but you'll get the idea.)
The other advantages with this setup are that I can also slide the whole thing forward or backward on the frame if I decide I don't like where it's at, and I can also remove it, if and when I sell/trade-in the trailer, and there won't be any holes left in the frame to turn off the next buyer. It did cost me some more, because you could probably get away with only 1 or 2 bolts on each side of the trailer if you drilled through the web.
1988 Coleman Sequoia - popup (1987-2007)
2012 Roo19 - hybid (2012-2015)
2016 Mini Lite 2503S - tt (2015 - ???)
2011 Traverse LT, 3.6L, FWD
2009 Silverado Ext Cab, 5.3L, 4x4, 3.73
2016 Silverado 2500HD Dbl Cab, 6.0L 4x4, 4.10