KleinGrrmpf

This IS the ultimate Expert Pack.
Split into multiple parts, some exploration in the beginning, then basic tech, advanced tech, magic, and then basically endgame, this pack slowly builds up. The pack goes throught all the well-known (and some less well-known) mods and uses pretty much all mechanics! But don't worry, if you are not a pro at every single mod. The questbook guides you through all of them with all the details needed! So in theory, even someone completely new to modded could complete this pack (even though some experience with an easier expert pack is recommended imo).
But yes, this is a hard pack. It is encouraged to automated everything! Well, not everything, but a lot! Mods and items build upon other mods and items and are intertwined so intensely, that it feels very complex (you might have seen the recipe tree in the big panel on the modpack page). But it is all made to feel organic with the 600+ custom items that are added. Some are ported items from past versions of mods, some complete a set where one item was missing, some combine features of different mods and some are just for gating.
Speaking of gating, the progression feels good! You don't get too op in the first few chapters of the questbook, but you are never underpowered against any mobs or need to grind anything for multiple hours. If you can get a new item manually, the ability to automate said item comes either pretty quickly afterwards or you are not going to need more than twenty of it.
Lastly, I need to mention the community on the Divine Journey Discord Server. There are people there who have played through the pack multiple times that are still on that server and help out other people. There is a forum filled with tips, tricks, setups, and explanations, if you ever get stuck on one.
And as of writing this, the pack is still tweaked and balanced every now and then, so any balance issue that comes up can be resolved.

That was the good part, now to the less good part. With all the automation you will want to do, it is quite resource intense. This can be mitigated if you spread out your base horizontally instead of vertically, but that isn't the most aesthetic way for everyone. So beware of that. Other than that, there is nothing bad I can say.

In conclusion: Better expert pack than FTB Infinity Evolved Expert, organic progression, complex and challenging recipes and recipe trees, beautiful custom items to tie mods together. If you like expert style packs, you're going to love this one!