I've not dealt with the USS, but I did have dealings with a similar scheme. Let me offer my observations.
First, you may not actually be given a choice about whether you join this scheme or not. Schemes like this are often compulsory. So while I applaud you for doing research, you may not have any decisions to make based on the research. If you are given a choice of different levels of opt-in your research may be useful.
Second (and you probably know this) defined benefit schemes are usually much better than defined contribution schemes, and it is usually worth opting in to them if you can. Being promised a definite retirement income, whatever the economic situation, and however long you live, is extremely valuable. It means the company is assuming some of the economic risk that you normally have to assume in your own pension planning.
So about security. A defined benefit scheme is actually a contract between you and the fund. You pay them so much money up front, and they pay you a pension. In that sense it is very secure. They can't say "our fund didn't do as well as we thought, so we will pay you less pension". They can't change their minds, or rewrite the terms of the contract, without your consent. You are guaranteed to get the pension they promise. However...
There is unfortunately one way the fund can fail to deliver the pension you are promised, and that is if the fund goes bankrupt. This can happen. I was a member of a scheme that did. It is rare. So how likely is it to happen?
With funds like these, size correlates with security. The USS has 400,000 members, which is huge, and 30 billion in assets, which is also huge. The chances of it being allowed to fail are very small. (My fund that failed had a few thousand members). Also in general employers are responsible for making up shortfalls in the fund. This is an obligation which can only be avoided by bankruptcy of the employer. The USS is used by many universities in the UK, and they would all have to go under for the fund to fail. (That's what happened to me, but my fund had only one small employer).
The UK also has something called the Pension Protection Fund, which is specifically responsible for taking over the obligations of bankrupt pension funds. In the unlikely event of the fund going under they would take over the assets and pay some fraction of the pension you were owed. Currently the fraction is 90%. With 400,000 members there is an additional degree of safety in that it is unlikely that a government would let the pension scheme of that many quasi-governmental workers fail. (No government wants 400,000 voters to blame them for financial ruin!)