The knowledgeable guys are more likely to be the ones who meet the trustees, not the members.
I worked in this sector in Ireland (though not recently). I never worked for Mercer, but I did work with them, quite a bit. They were very, very good.
But that's at the level of actuaries, etc, advising trustees on investment strategy for funds of many millions, or advising employers on pension scheme design. Providing individual advice to members on their personal situation is at the other end of the business and I wouldn't expect Mercer (or any of the other large providers) to be oustandingly good at that. The reason is that if you're oustandingly bright and highly competent, working for a large consultancy you will find yourself advising the largest clients, where the most money is to be made.
That's not to say that the advice given to individual scheme members will be dreadful. Fortune's experience was bad and, I would hope, not typical. From a consultancy like Mercer you should expect at least basic competence. Plus, they should be good at devising and delivering solutions that, for the typical and common case, will work well.
If you want advice tailored to your personal circumstances, and certainly if your personal circumstances are at all unusual, as Zion2022 says you're most likely to find that at some of the smaller, independent firms. The problem is that you're not necessarily going to find it at all of them, and it can be hard to find the right one.