Interestingly enough in my latest Numbers Aren't everything I have experienced that ARM+MNT is better at attacking hill than 2MNT.
Agree. But I wouldn't waste MTN to attack just hills anyway. INF/ARTY can do that just fine.
Combined arms compensate terrain loss and due to decreased softness and ARM stats ARM+MNT has much better firepower and toughness.
Yes, the CA bonus can do that, as well as mitigate out of supply penalties (why you get the CA bonus when you have little or no fuel is a mystery to me). But if you put ARM and MTN together, we're talking about a formation that now needs fuel. And that ARM is wasting all its wonderful speed hanging out with a bunch of mountaineers. Doesn't it have something better it could be doing? Like running armored warfare on the plains?
HARM could be also considered as good option for adding it to even simple INF. I have not tested it but two CA units 2HARM+2INF are probably better than 4HARM and 4INF.
Well, I wouldn't use 4xINF anyway.
It's not a completely stupid formation, but it can certainly be optimized a lot more.
Here's the thing about HARM with INF. Let's look at SA values first (all values are 1941).
A HARM brigade has an SA of 6.33.
An INF brigade has an SA of 4.4.
An ARTY brigade has an SA of 6.
Okay, let's build some divisions.
4xHARM has a width of 4 and a total SA of 25.32 (6.33 SA per width)
4xINF has a width of 4 and a total SA of 17.6. (4.4 SA per width)
HARM doesn't give that much more firepower than the INF division, but the softness (10%) versus the iNF (100%) means it takes less damage from most non-armored units in the game. If they didn't bring TDs, ATs, or CAS to the party, the HARM bulldozes INF out of the way (it takes so little damage from INF/ARTY divisions as they engage in combat), but it does it expensively. In terms of IC/days versus SA, INF wins. In terms of logistics, INF wins. But the pure HARM division doesn't really care about your SA as it slowly, inexorably moves through the province on attack or holds the province on defense.
Pure HARM also gets hard on soft bonuses in combat, killing more ORG on those soft units. How important this is in the big scheme of things (given HARM's other attributes in this kind of warfare) is open to discussion.
Now let's look at INF/ARTY.
My standard 2xINF/2xARTY division has a total SA of 20.8. (10.4 SA per width)
This division is WAY ahead on terms of concentration of force. If you are only looking at inflicting damage on soft units, this division hurts soft units way more than either of the other divisions when used in stacks.
Again, HARM has a little more per division, but the INF/ARTY division is even more cost efficient for that SA. It still wins in terms of logistics over HARM, but again the softness is the big factor here. HARM has low softness, while INF/ARTY has 100% softness. Again, the HARM is an expensive way to achieve certain kinds of results. The INF/ARTY division blows away other divisions through superior firepower. HARM turtles it's way through combat with soft units, laughing off damage that would seriously threaten the INF/ARTY division.
What about INF/HARM?
2xHARM/2xINF gets the combined arms bonus (let's be generous and assume the additional bonus to CA from the appropriate doctrine) and has a total SA of 27.898 (6.9745 per width).
Even with the CA bonus, this division still has less SA per width than INF/ARTY! It also only has a softness of 55%. That means that this formation, while CA, does not get hard on soft. It uses fuel and more supplies than INF/ARTY, and it was way more expensive.
What about HA?
4xINF has a HA of 6.88 (1.67 per width).
4xHARM has a HA of 29 (7.25 per width).
An ARTY brigade as a HA of 1.6 per brigade.
Unsurprisingly, HARM comes out on top in all categories here. a 4xHARM division has 4.2 times the HA of a 4xINF division OR an INF/ARTY division. The INF/ARTY division has weak HA even when looking at width. HARM still has twice as much HA per width as the INF/ARTY formation.
What about INF/HARM?
2xHARM/2xINF has an HA of 23.192. (5.798 HA per width) Assumes combined arms bonus
This division can fight armor more effectively, but it still sucks compared to 4xHARM.
What about other ways of getting the CA bonus with INF?
Let's try INF/TD.
Each TD brigade has 2 SA and 7.2 HA at 1941 techs. It has zero width.
2xINF/2xTD has a total SA of 16.64 (8.32 SA per width) with the CA bonus (including the extra CA from doctrine).
It has a total HA of 23.062 (11.531 HA per width) with the CA bonus.
This formation has better SA per width and way better HA per width than the HARM/INF division. It costs less, still gets the CA bonus, uses less supplies and fuel, and can be built using starting techs for some countries in 1936. It's even cheaper to upgrade in terms of both leadership and IC cost. It's more expensive than using AT brigades, but you lose the CA bonus there, resulting in overall less efficiency.
Basically, I can't see any advantage to stacking HARM with other brigades in a division. Every other division configuration either has comparable firepower for less IC or has better SA/HA per width. Putting HARM with other brigades in a division just dilutes its only real advantage: softness.