Basically it's very difficult to tell which items are best on which champion and which items do nothing. Champions that are manaless on SR (Kennen, Akali, Yasuo, Garen) do very well with mana items on TFT for example. But champions who normally use mana like Kassadin are manaless on TFT so you shouldn't put Tear items on him. At least I think? It's very opaque.
I'm sure there will be build guides soon enough as people try more things but one of the reasons that the Guinsoo's hypercarry build became so popular is that it translates directly from SR. People understand you should put guinsoos/iedge/rapidfire on an ADC. It makes sense.
What champions are using spells vs not having spells at all is not clear in many cases.