Have had the 950 XL for about 5 days now, and only had one reboot, and one time where the phone was getting hot in my pocket for no reason, so pretty stable overall.
Today though I got a SD card, a Kingston SDCA3/64GB U3 card, and reboots just every time i write/read to/from the card, have set everything except the apps to use the SD card, if i take pictures and safe to sd card sometimes it reboots, but it happens every time i try to take a movie, and if i play/delete that movie, reboot, transfer files to SD robot. Just in the past hour about 20-30 reboots.
This is a joke, in 2015 this is an issue, can remember back in the days when Nokia 920 came out, it just rebooted all over the place, and still now years later reboot is still a major issue on Windows Phones. Would much more like if they did not support SD card, and maybe sell with more in built storage, if the phone just reboot overtime the SD card is in use.
Have read that some who put in a slower/older card to the phone, have resolved the re-booting issue, tried some 16GB ultra card from Sandisk, but the phone would not even see the card.
Anyway have been on AC wifi all the time I have had the phone, so this have not been an issue here, the SD card read/write is a BIG issue though, I hope Microsoft will address this ASAP.
Really love windows phone, but needs to work on stability.