Sastopher

I've spent a lot of time strategizing with this list and have played against it in tournament (as Skaven). It's a glass cannon that wants to strike hard enough that the enemy is crippled and can't hit back effectively. Generally this means focusing high-value targets with mortal wounds, such as heroes and synergy pieces. As a sort of corollary, it does very poorly against horde armies with lots of cheap wounds, because it means your rather expensive mortal wounds aren't getting good value. Lots of models also allows deployment strategies which keep you out from reaching the key pieces of their army, either by bubble wrapping or by spreading really wide. This concentrated fire strategy also means you are kind of all-or-nothing on objectives. If you don't wipe your opponent out, it's hard to maintain objective control across the mid and late game. My take on theorycrafting the list is it would feel oppressive to non-competitive players, but perform poorly in tournaments where opponents are familiar with counterplay options.

There are some competitive lists out of Moulder as well. Giant rats are a cheap and high-damage battleline unit that can cover for stormfiends (which are also moulder) and a cabal of packmasters. Their only deficiency is terrible bravery; at full strength they do pound-for-pound some of the most damage of any battleline unit (3 attacks at 2s and 5s is 5/6 of a wound per rat). I usually take 5 packmasters, which I know is above the commonly-recommended 2. You still get really good returns out of the 3rd and 4th whipping (as it still increases the odds of exploding attacks), the extra charge range is often really useful, and packmaster redundancy in an all-moulder army is rarely bad. In objective play, I often split them into two groups to cover different ends of the battlefield. The rest of the list is allied units round out the list, providing mobility and range. An arch-warlock, or gutter runners, or a few weapon teams are all interesting options. It took a bit of effort to get 120 giant rats but there are some plastic sets that come with a few dozen rats you can sub in. They're often sold as bits at a decent price too.