Subs disrupting convoys can make a real contribution to victory. Two sea zones in particular are massive convoy opportunities - sz97 & sz6.
sz97 can be used to destroy Italy’s economic contribution. In the games in which I participate significant allied effort is expended towards achieving the air and naval supremacy in the Med that allows Italian DD builds to be swept away by the US. The US is hampered by fighter scrambles, so retreat as soon as any Italian DD’s are wiped out is made possible if UK subs are available to move in during their non-com.
Note the need for air and/or naval supremacy if your subs are to survive. sz6 is a good example of this, as J’s build potential and opening air and naval power severely hamper allied attempts to gain traction. But if J’s fleet is engaged beyond reach of sz6, perhaps attempting to defend the money islands or take India, the opportunity can present itself. This has occurred in a proportion of the games in which I am involved, so not as common as sz97, but not uncommon either.
A lesser example is J subs off Ceylon - which can make a real difference given UK Pac’s low income.
So yes - lots of success - in the right circumstances.
By contrast, G is unlikely to be able to maintain air and/or naval dominance in the Irish Sea, so is likely to lose subs there attempting convoy disruption.