"How finicky is notion with stylus input?"
Sadly, Notion Handwriting is finicky - period! You can´t use Notion Handwriting at all !! A bug was introduced during spring 2019. It has not been corrected. It maybe will be - some day, (that´s purely speculative on my behalf) I´m not holding my breath..
NOTION / HANDWRITING (one of the selling points, I imagine) DOES NOT WORK IN ANY USEFUL WAY.