Everything you always wanted to know about Fusion ammo:
https://ammo.com/bullet-type/fusion
I also had questions about it, specifically for .338 Federal. I had an impression that it was 'budget' ammo (and it *IS* generally fairly priced, less expensive), but after talking to other people more familiar with it and doing a bunch of reading, I've come to understand it is actually a very capable bullet. Accuracy out of the box seems to be consistently excellent as well.
I haven't used any yet, but will be getting my rifle next week (finally! Yay!) So I will be trying it in 200 gr along with Power Shok 200 gr, 180 gr Accubond, 200 gr Trophy Bonded Tip, and 210 gr Partitions that I have managed to source.
I wouldn't be shy to use the Fusion on anything, but I am more partial to the 3 premium bullets I've sourced. Whichever shoot the best will get the nod.