I am just a hearing wearer and can only comment on my own experiences.
NHS hearing aids can be excellent, and at the moment I think there are about 3 major brands that they provide in a re-badged form - these are Siemens, Oticon and Phonak. Usually, the hearing aids are about 2 years behind the current latest offerings, and they are perhaps the mid segment range for that product. Although for super power losses, the Phonak Naida is re-badged as a Phonak Nathos - this is a premium aid.
So, if you do not wish to go private, perhaps have no money, then you can get an aid, which is a bit like buying a 2 to 3 year old car perhaps - with not the latest uo to date features, but it will still be excellent, even by today’s technology comparison.
The disadvantages are the the NHS do not supply in the ear hearing aids. So you will not be able to get a CIC (Completely in the Canal), and ITC (In the Canal) hearing aid. I think they may supply ITE (In the Ear) hearing aids - which are much more noticeable - but, only if the hearing aid wearer has a particular problem that precludes them from wearing a BTE (Behind The Ear) hearing aid or RIC (Receiver in the Canal) hearing aid.
Some NHS hearing aids do provide bluetooth functionality - like the Oticon Spirit Zest C (Communicate) version - but from the information I have seen on forums, this is a bit of a post code lottery - it is not guaranteed. Certainly, if you go private, you will be able to get bluetooth - even on smaller hearing aids.
With respect to private aids, there is also a greater degree of customisation and personalisation. And you can also get IIC - Invisible in the canal hearing aids, that will certainly not be available on the NHS.
You may also hear arguments about levels of service with respect to private v NHS. I haven’t personally had a problem with going back for repeat visits to the NHS, but I have heard some people claim that their time is limited - but then I’ve heard counter claims ! So I’m not sure.