https://www.bbc.co.uk/news/articles/cy82pxlmmyno
If you've ever been through a DWP PIP assessment, you'll know how their "medical professionals" are there to try their utmost to prevent you from being awarded PIP.
The trouble with Fit Notes is that many GPs will sign you as not fit for work with no deep investigation, and some will do so without ever seeing you, mainly because they think they are not paid enough to provide the service.