okay had a day to organise my thoughts, and while some have been shared and discussed already, here's how I feel about the list in all honesty
I like it, it's a fun list and I think the AC has done a great job keeping the theme and focus and can't fault his work
however I find it to be very very weak, the combination of all the big stuff being move 15cm coupled with int3+ really hurts things, the slow move means you struggle to get to objectives (against a canny opponent you'll almost never get DTF with this list) and also you're limited in moving to get good firing angles on stuff
the list has lots of shooting, but unless you like blast markers, you're almost always doubling which greatly reduces your effectiveness, the AMTL list needs the firepower as you're coming from such an activation deficit you need to be able to destroy enemy formations at range to level the playing field, currently the OGBM list struggles in that role a great deal more
the other big issue is that most of the assault weapons available to the list are CC weapons which further compounds your puny 15cm movement woes.... the much discussed reinforced boiler upgrade is such a no-brainer choice in the list if you want to get anywhere...
other than those more general points, some specifics
-Supa Stompas: I think these guys could do with a points drop, at 325 points they are expensive, and for my money not nearly as useful as the warhound in the AMTL list, they lack the speed and flexibility of that unit, and when broken they seriously struggle to rally.... unlike the hounds... I'd suggest dropping them to 275 as they will almost always be upgraded, they're just not worth it currently
-Reinforced boilers: These are an auto-include for most lists which is cause for concern, Jimmy has suggested they be replaced with a 'push em 'arder ladz!' army rule which means all gargants/stompas can get the 5cm move at the risk of a backfire, I much prefer this option and see no issue with it being specific to this list
-int3+ : this is a real sore point for the list, it penalises you for trying to hunker back and shoot and wastes valuable warboss rerolls which would be better served trying to rally the broken supa stompas
I'd suggest it needs some modification, perhaps adding +1 to sustain fire actions (making them less of a horrible risk) or perhaps a bonus when within 30cm of your warboss (it could even be an upgrade, 'Uge Shouty speakaz' or something, warboss only, all friendly units add +1 to sustain fire, advance, overwatch, marshall and march orders when within 30cm of the boss gargant)
I really like the list, I do, I've specifically painted up extra gargants to use with it, but unfortunately I'm struggling with it at the moment and that makes me a bit sad!
