Could well be they work fine. When I worked at the building place I used to see a lot of complaints between customers and tradesmen about them. Companies like Envirovent do a franchise type thing where you become an approved fitter for their products then work as a sole trader but use their branding. It means there's lots of trades out there which suggest them as a solution for every problem as all they do is fit PIV boxes. Much like the damp proofing companies which suggest wall injections as every solution.
TBH most people i know with them are geeks/nerds who want to know how they work and research what they need. if people are cooking/washing/using bathroom with no other moisture management then they will not work on their own.
Could well be they work fine. When I worked at the building place I used to see a lot of complaints between customers and tradesmen about them. Companies like Envirovent do a franchise type thing where you become an approved fitter for their products then work as a sole trader but use their branding. It means there's lots of trades out there which suggest them as a solution for every problem as all they do is fit PIV boxes. Much like the damp proofing companies which suggest wall injections as every solution.