The Elysium modpack contains dozens of mods, which alter pretty much every aspect of the game in some way or another, ranging from minor enhancements and tweaks to major redesigns. It's always important to exercise caution accordingly.
As with all mods, a certain amount of 'buyer beware' should be remembered - we've only heard positive things about the Elysium modpack and creator TitansBane, but you are altering your game and computer regardless. Elysium is also designed to be fully compatible with the Creation Club (though it's not mandatory that you have Creation Club content added).
The Skyrim Elysium modpack is a massive general upgrade for Skyrim that includes hundreds of new mods to add new quests, items, graphics, characters and more, some of which expand into whole new locations and additions to the world.