I'd be willing to accept smoke rounds as the start, if to compensate for the fact the ISG is terrible versus garrisons and actually harming the structure. If they can be smoked off, then the ISG can at least help OKW's massive issue with garrisons and defensive positions when using the Battlegroup HQ.
The Battlegroup HQ, as it stands, needs considerable help as an actual tier structure rather than mainly being a beefier Forward Assembly with healing.
And in general its worse against anything not emplacements and mortars can also damage vehicles, the ISG just as a tad easier time hitting them due to its arc, but it's a very minor thing as it has much worse AOE.
ISG is terrible versus garrisons, but Flamennades are perfectly fine, for example. Wuhrframmen is ubereffective against garrisons - just crushing buildings with troops inside. So, what problems may you have with them?
Idea of "smoking off" garrisons is a bit wierd, never saw anyone, who would smoke it, instead of attacking and punching units out of there. Just throw there 1-2 flammennades and it will be 100% clear, no need to smoke it or do something more, seriously.
Battlegroup HQ is also fine, if you will look at todays faction design. You can get it pretty fast, cos it costs not so many. With that you getting 5th unit in T0 (is there any other faction, which has so many units in starting tier?), you getting all kinds of units, all kinds of counters for everything - from infantry (HMG, infantry, FlaK HT(may sound like a joke, but still)) to tanks (Puphen, SPioAT, Volksfausts). Even without tiers OKW has more, than any other faction, with T1 they also getting very effective indirect also. Personally, I see no reasons to build T2 for myself, cos light vehicles suck, Wuhrframmen costs too much and I'd prefer to save my fuel for T3 and a bit early comming vehicle core of OKW - Pz4s and Panthers. But, that's my own build for games.
In any case - Battlegroup HQ is fine, with that OKW very fast gets everything they need. Getting higher tiers only opens for you tanks. Yet I still think, that if we need to pay for medics additional 100MP - why not to let FRP on BGHQ cost 100 MP less? It all just costs too much...
And in general, it's not worse than usual non-doc mortar. Mediocre accuracy, fine damage, never saw, that mortars can damage medium vehicles like LeIG or packhowie and it's AoE is also fine. It's not so big, as mortars, but... mortars has lesser range, lesser penetration. It is an exchange - range and power to areal damage. Balancing. For non-doc and early coming field howitzer (pack howie comes later, costs more and less mobile, and for that - more effective, so uncompareable) LeIG does it's job veeeeery well. It counters enemy mortar teams, enemy HMG teams, able to snipe from fog opponent's vehicles. Has problems with garrisons, but as I said - flamenades, wuhrframmens, sneaky encircling tactics - all of that can be used effectively against them, so use it. And no need to make OKW's gameplay easier, it already became so with getting non-doc HMG. Save some work for OKW players and some uniqness of that faction. That new patch almost ruined it already. Add smoke and it will be not 70% copy of Ostheer, but all 80%. Is that, what we want in faction designing? Definitely no!