Is there a reason exhausts arent solid mounted?

[Jack91]

Hi all,
In a couple of weeks I will be attempting to make a twin 2.5inch exhaust from the extractors, dumped at the diff, for the fairlane. Reason being is that I'm comfortable welding it myself, and I cant justify the ridiculous price of buying one premade, when all it will take is 2 bends and a slight mitre or two. Now, seeing as this is a very budget build, for time and money, its going to be a lot easier for me to solid mount the exhaust off rails/bars rather than buying and bending rod and rubber hangers. I figure I will have a flange at the extractors, and another before the curve over the diff.

Unsure at this time whether I will use stainless or mild steel, it will depend on the price difference. Obviously stainless is going to be a little trickier to weld, and should withstand heat better without distorting as much as mild steel. However, I'm doing it on the car while its on stands, so getting the torch into the tight areas to tack could be tricky.

Any advice greatly appreciated