I'm mounting some (very) heavy duty TV wall brackets. The screws and
plugs
came with the brackets, as did the drilling/fixing instructions.

I don't think I have ever used screws and plugs that came with something.
They are almost by definition, utterly unusable.

Much better however, would be to use a system I know works well: Unifix
plastic wallplugs and traditional woodscrews.

Indeed, but you can use moderns screws, too. The failure in the supplied
parts was because they were bought by the manufacturer for 0.0000000001p and
they got what they paid for.

Christian.