Formation should be used based on your opponent! I always try and see the advantages and disadvantages I have against my opponents. Right now, my league is very week except one guy that is neck in neck with me (he is my friend and also a token buyer). So basically when I play him I always try to see the best formation to counter him. Same thing in the CL as I have reached the quarter finals and things start to get tricky so you need to put your best formation out there but keeping in mind what formation your opponent will play. One thing I saw a couple of times is that you prepare your team to counter your oponen't s formation and in the last five minutes before the game he changes it so then you have to improvise
These are called very effective players