Just so it is said, the bug I am about to tell about I have only found in one of the newer modpacks (Atonement: Sins of the Past, which for some reason is in the "mods" section of the servers).
Now, the issue is one which I can see happening on many other servers/modpacks as well, and it is that Aternos cannot generate the custom form of world generation that is required by the modpack. Furthermore, it cannot load a save made by the client, and ends up having an error loading the items. What this means is that you cannot play the modpack the way it is meant to be played, and if you want to play it on Aternos you must go with a default world generation, totally unfit for the pack, to be able to play on a server.
The serverlog of a loaded world with proper world generation is attached, though it is in a .txt because if not I get the error "The message is too long, must be under 10,000 characters"
Note that even though it says there are many issues in the log, when going with SP you can easily start the same world
If it is possible to have Aternos server use the modpack to do the world generation, this issue might be solved.