I completely agree with Arcane1729. The difference of difficulty between different universities especially in STEM subjects is huge. I have immense respect from people like Arcane1729 who study mathematics at respectable universities.
I studied economics, not a STEM, degree but surely most people would classify it as a respectable degree. I initially enrolled at an ex-poly due to bad A level results. After one year I realized I was waisting my time. I went back and resat the exams and I was accepted at a respectable university.
The difference between the respectable university and the ex-poly exams were huge. Even the levels of mathematics was way higher at the respectable university.
So bottom line, yes university choice does matter and yes the same subject at different universities can have massive differences in terms of difficulty.
In short, if you can, try to avoid ex-polys if your grades are good and go to well reputable Unis if you want to learn your science really. The rest is just a waisted of time, gaining sub standard degrees that will get you nowhere in substandard universities. I am exaggerating a bit here but I am trying to make a point.