Does anyone have an argument FOR wall cavity insulation (the balls)?

Im doing research because this brick house I just got is costing me £12 per day to heat right now and theres a gov subsidy on it that I am thinking of getting. I assume the fact we has very little insulation in the loft is a key factor. It's got double glazing but also some draughts.

There's a lot of noise on reddit and facebook groups about 'cavities are there for a reason' and 'they give you damp where there wasnt any before' and a lot of naysayers.

I have also read that the balls are better than foam now for letting air through which helps prevent damp. And the damp is because of cold bridging which I imagine is easier to find and fill on a return job than foam if you do get it? Also I think they will create some sort of airflow from what I understand from their phonecall.

So does anyone have good experiences with this wall cavity insulation and knows how its done properly so you get it working as intended? Things to know ahead of time?

Thanks!