It's not that hard to see through the RG unis. First, research into them. They aren't specially selected universities. In fact, they are 100% self-declared. There isn't anything inherently special about them and while in general they are highly ranked overall, there are many non-RG unis that easily compete with them (take places like Bath, Surrey, and St Andrews for instance). There isn't an external organisation who has carefully picked which unis deserve to be RG.
Second, the urban myth that higher ranked unis have harder exams isn't true. If it were true then exam difficulty would fluctuate massively year on year as universities jump around the rankings regularly. The ranking your uni is at when you start your degree will probably be quite different to the ranking it has when you graduate.
Third, since starting a doctorate degree I've been sent to study at several unis who specialise in different areas so I can supposedly receive world class learning from each. I've studied modules at a total of four different unis, of which one was RG, and it is unfortunate to say but the RG uni was the lowest quality out of all of them. Let me show you an exam question from a Master's level exam paper from 2014 aimed at MEng (engineering) and MSc (science) students.
This is from a 100-mark paper (2 hours). Read it carefully. Now, I might be biased as I come from a physics background and so I've encountered many equations in my time, whereas engineers are more used to wordy questions, but this question seems ridiculously easy. You have an equation, and the terms in the equation are explained. It then tells you to calculate the first 4 sub-band energies (i.e. n=1, 2, 3, 4. Anyone who studied this module would know n was the sub-band level). You are given h-bar in a separate handout. So you have every single term needed for the equation and the equation is already arranged into the correct form. All you have to do is put the numbers in for 4 marks.
And then once you've done that you put another number in for another 4 marks.
And then after that you repeat once more for 4 more marks.
You've now gained over an entire grade from something that a 10 year old could do.
Welcome to the way RG unis assess their students.
I'm not going to name unis here, and in the RG's defence this paper was quite difficult to get a high grade on, and yeah it was aimed at Master level engineers who might not be very experienced when it comes to putting numbers in equations. However all others unis I went to and sat exams at the exam papers literally contained zero 'easy' questions. It was all brutal challenges from the very beginning to the very end. This RG uni seemed to not want any students to fail so they put in a few easy questions so everyone could get up to the pass mark. Certainly makes them look better if fewer students fails.
So yeah, the rumours about RG unis having harder exams than non-RG ones... load of crap. At best they are equal, at least when it comes to high grades.
What I have found from my travels to many different unis is that each one has its own feel and personality. The town or city it's in does too. Both these factors need to be taken into account when you are choosing your uni. You aren't missing out if you don't go to an RG uni in the slightest. They are really overdone and good unis need to be chosen by someone external, not by unis self-declaring themselves as special.