Like I said guilty pleasure that one, as for the story it wasn't really well done and even for me towards the ending the novelty of it was begin to wear off, but I still enjoyed reading it. Seeing what sort of OP abilities the author will try to come up with with all of it meaning nothing in the face of "the end of all fates". Definitely a few moments after the ending of the Anime was pretty nice and the backstory was interesting, just nothing ever gets flushed out, because not really as much of a story but more about power scaling and he is the pinnacle of them all.
One thing for sure however, it is hard to imagine how that world even survives but according to Sion the world was much safer until Yoigiri arrived which is quite funny.
Honestly hard to remember the Assassin anime barely at all, personally think that show had so many flaws to it and even if they got it right I still think it would be just mediocre. The Water Magician is a little different, it was a decent watch for a Season 1 but I was deciding between 6-7. I think Season 2 if it gets one will be the deciding factor. Just the way the stories were told, one is close to unwatchable and the other one more mild.
Also, finished my first ever LN experience was quite something even if the show itself was mediocre (but to me guilty pleasure) somehow reading source material was honestly very fun.