I have to disagree with this post. In late game the complete field is plastered with yellow crater holes and you can build green cover with your IS wherever you wish. IS have crazy vet and win against obers (green vs green cover) by a big margin when having 5 men & brens.
I don't see good players doing mortar or isg spam, since its not an issue to counter, as you can push usually players who go heavy on indirect fire away or go double sniper.
i actually played against a decent brit player who used IS since i made that post and i will have to revise my opinion of IS somewhat. they're definitely strong with the 5 man squad and bren and will certainly beat another basic squad if they have cover. they're still really vulnerable to AoE though and on certain maps you don't always have cover everywhere if you're trying to flank, which is a mark against them. with all that said, i think IS performance only shines if you have both brens and the 5 man squads though, making either single investment on IS generally a waste.