This is about １０分で読める伝記? I’ve so far only read the first two stories but I was surprised how much fun and how easy it is to read them. I think saying that they use nearly no kanji is just wrong. If I open a random page there are always lots of different kanji. I’m not a fan of hiragana-only texts myself but I think these books are well balanced.
On the other hand I wouldn’t call the use of onomatopoeia excessive and the ones used are pretty common and useful.