Hi Pycho! Don't worry.
You'll still be using texture groups. The point of Managix is to combine all the confusing and manual work of GameMaker functions into an intuitive API, giving you greater control over when to load, unload, or even keep them in memory.
Each Managix_Resource can be a texture group containing sprites. And a special thing about Managix is that you can load sprites without freezing the game, and there's even support for loading things on-demand - that is, while you are already in the level, without freezing the game.
Without Managix, it's much harder to know when texture groups have finished loading - also harder to know the loading progress, and you still have to deal with GM's async functions which are cumbersome to handle manually.