Very strong in a lot of ways

Meatballcraft is an ultra expert (GTNH, DJ2 tier) pack and one of the few packs I've ever seen to demand that you do everything. It is a 1000+ hour pack that expects you to automate everything possible to automate, with some Modular Machinery to help where you can't.

Strengths:
Breadth - when I said you do everything, I meant it. Nuclearcraft underhaul Molten Salt Reactors, bees, trees, Bewitchment, all of it.
Lore - How often do ultra expert packs write full lore around the world and progression?
Custom Content - the dev has added immense amounts of custom content. Puzzles, new structures, new dimensions, custom weapons, and more.
Documentation: MANY mechanics from many mods are explained in game in JEI toolips or tabs, even sometimes where ingame documentation (Lexica Botania, Sanguine Scientiam, Thaumonomicon, etc) exist.
Combat - Combat in this pack is heavily rebalanced. You can very easily get 30 damage swords in a variety of ways fairly early and other methods of dealing damage are very strong, but you're never overpowered. Mobs scale as you progress, bosses have more hp and armor and hit harder, etc.
The Dev Himself - Sainagh is VERY involved and very invested in creating new content. He is very receptive to feedback and suggestions and the pack's rough spots have been smoothed out a lot through playtesting.
Balance: Combat, crafting costs, and crafting chains are very well balanced. The pack presents a challenge at all points and offers the option to play and build how you want, as opposed to strictly following a quest lined progression. In the later chapters with more custom content, the content ends up more linear, but still has room for flexibility and variety in how you play.

Weaknesses:
Length - this is a 1000+ hour pack. It is VERY easy to burn out on. The bulk of the content STARTS where a lot of expert packs end, and you won't have reached anything from Advent of Ascension, Draconic Evolution, Nuclearcraft, or dozens of other mods.
Stability - for the scale of the pack and how much is in it, it runs EXCELLENTLY. However, with a pack as large, long, and complex as this? Your base will eventually grow into a server crushing behemoth. TPS management is critical from minute 0 so that you don't have to rebuild things. This isn't a pack that can be hosted on a public server for anyone to play.
Documentation: EVERYTHING is explained. It gets excessive, especially for an ultra-expert pack like this. Generally, you'd expect the target audience of an ultra-expert pack to come in with solid background knowledge of modded Minecraft, where knowledge of AE2 and Botania rune crafting are commonplace.
Balance: Meatballcraft hands you ProjectE in the first chapter, 50 hours or less, I'd wager. Some people may not like this. This pack is not one where vast quantities of resources are the struggle, but rather where automation and progression require complex chains and new mechanics. This is not for everyone.

All in all, it's a very strong modpack that hopefully has sticking power and appeal well above its current playerbase. The pack's content is very highly polished and the depth of content should present a pack that's challenging enough to attract the most seasoned and jaded players.