A certain rabbi said that Megillas Ester written in Ksav Arizal should not be used in nussach Sefard shul because people won't be yotze with its reading. What makes Ksav Arizal so different as to make it's reading unacceptable to some?
Modern "Ari script" has the right head of the Tzadi facing to the right, while modern "Beit Yosef" script has it facing left. Historically, the head has been written facing all sorts of ways (even upwards) even in Ashkenazi lands without qualms.
In the early 20th century, the Chazon Ish famously [allegedly] was concerned that the modern "Ari script" way was invalid, but most rabbis defended the viability of the practice. See R' Aryeh Friedman's book on the topic Tzidkat Ha-Tzaddik.