They are for all intents and purposes the British equivalent of the Flame-thrower.
Instead of random crits ya got random accuracy, but the base principles of an anti-cover/anti-building weapon still apply.
rifle only get damage bonus against garrisoned units.
The rifle nade merely ignore light cover.
Heavy cover still provide directional cover, so the grenade have to land over the sandbag.
If you don't like having to babysit a LT then I would suggestion roll a heavy rifle grenade build as lt doesn't provide much of a bonus to the nades. Maybe vickers with rifle nade.