It is not that anitum is difficult to grow, it has a reputation of being a slow grower. Slow is a big difference from difficult.
It is an extremely fast grower in the wild for certain.... Look at the photos of the wild ones from the wild, they have a blooming growth, one two NBS, 3-4 smaller ones, and sometimes start on top. I would say they can flower yearly... Some are huge, huge clumps too.
They are big plants, the size of roth, sometimes much more. So it comes back to the other discussions, people don't know how to grow them at all, and the survivors are bonsai.
'seedlings' of 10-25cm sold here and there have one source, and they are anitum x randsii. The leaves even do not match anitum at all, they came from a German nursery ( honorable) who sold them as the hybrid... Sellers though it would do the job, and a better profit, so here we are, but as of now do not waste your money on them...
It comes back to the feed, they are heavy feeders, ammonium, and not too high pH, like most plants that are growing naturally in fern roots... Live fern roots has a pH of 5.4-5.5. When they are dead or cut off the pH crashes to 4 or sometimes below. That's why many people mistake that fern root environment is a very acid one...
They need to be kept moist to wet constantly too, more on the wet side actually.
As a certain fact anitum hates nitrates too...