It is a way to get the most salient points which stand out from the evidence down in a readily digestible form, though one which is not as defended against cross-examination as the full formal report. It can be good value for instructing solicitors, and as long as well all understand that you get what you pay for, it can be a good place to start.
What you do not get from these kinds of notes and advisory reports is something you can submit to the court, and you also lose nuance. Nuance can be important, but if you case is a slam-dunk for either side, it's best to know that as soon as possible.
I had a new enquiry this week for a new expert witness case in the high court, but most of the new enquiries at the moment are for proper engineering, troubleshooting SAF plants and designing an aeration system, amongst other things.
There isn't that much new in sewage treatment, as I was telling some lawyers yesterday. The activated sludge process was developed over 100 years ago, by Lockett and Ardern in Davyhulme, Manchester (UK), and the first large scale trickling filter was installed in the US around the same time, (at least according to Americans) based on the development of designs by British engineers.
There may not be much novelty, but we are still working on fully understanding the technologies we have. Sewage treatment is very complicated. You might be wise to save money by reducing the scope of your instructions to an expert, but you should never compromise on the quality of your expert. There's no such thing as a cheap engineer!