Most of the “reasons” not to do it are myths in my opinion.
The only reason the cavity approach is used, is because it’s far cheaper to build a Brick and block wall with a cavity, than it is to build a double skin solid brick wall (like they used to do)
Personally I would say go for it, it’s not as good as other forms of insulating your house, buts it very cheap and it’s certainly better than doing nothing at all.
I would ring around and ask what fill different people are using, try and get someone who uses Polystyrene balls rather than mineral wool.
Polystyrene fills more completely leaving less voids, and it doesn’t slump over time like mineral wool.
Far more effective in the long run.