I've noticed people on TSR are often quick to criticise RG universities, specifically those which aren't typically thought of as being in the UK top 10 (so basically all of the Redbricks plus Glasgow, Southampton, Queen Mary, Cardiff and QUB).
The main criticisms are that there's an awful lot of marketing involved, that they are a 'self-selecting' club and that some are stronger than others. These are all true, but that doesn't mean these universities are no good. Two things can be true at the same time.
The truth is, even the 'worst' Russell Group universities are pretty decent. Even the 'lowest' ranked ones are pretty high on global league tables (all of them usually sit within the top 200 and can actually claim to being part of the global top 1%, unlike supposed rivals like Reading, Sussex, Lancaster etc. which make similar claims), are extremely well-endowed and unlikely to face any sort of financial trouble in the future (which is extremely relevant in current times).
Yes there are other good universities, but it's only really a couple of exceptions (St Andrews and Bath) which are on a par with this group. If pressed, most people would probably rather attend an RG uni than some non-descript plateglass or an ex-poly.