Drop rate is a real pain, I have the firey warhorse after farming it casually over a few weeks. I have the death chargers reins farming it to waste time before a reset on my rogue. I just snagged riens of the drake of the south winds (TO4W). Addtionally back before mounts were account wide i also managed to farm up five seperate white hawkstriders. my rogue was by far the luckiest having had 4 at once, (one learned, one in bags, one in the bank and one in a loot window). I managed to snag the raven lord after a week of farming everyday on one toon. I rolled a 96 a few months ago to win Spawn of Horridon, I snagged the blue protodrake in my first ever utgarde pinnacle heroic run (actual first herioc dungeon at 80 that I did on my first toon ever). I farmed ashes doing 4 runs a week for maybe two months straight. I've also farmed firelands maybe a grand total of a month's worth of attempts and actually saw the rag mount drop (it was ninja'd though). I could go on for another paragraph explaining my mount luck but alas that is not the point of this post. .
I've been rolling on SHA every week for at least 2 months across multiple toons (varying from 2 toons, to five last week) with absolutely no luck. By no luck, i mean that only one person on my home server (stormreaver) has the mount (that i've seen riding it anyway). That means that in the past 2 months if i was averaging 3 kills per week and seeing at least 120 people a week roll on it twice (I assume they are bonus rolling) thats 240 unique attempts that i've witnessed per week and not a damned person i've come across has had it drop for them (don't tell me that they wouldn't link the soulbound riens in chat or at least the spell). However i'm gonna keep trying every week.
I almost feel like the sha mount is still bugged in some way, (it was bugged earlier in the expac to actually never drop, for anyone that didn't know). Unless the drop rate is just this low. Maybe like everyone else i just have terrible luck with certain mounts but not others. Still i wanted to at least see proof that this sha touched cloud serpent actually exists as soul bound riens, lol.
TLDR: everyone has terrible luck with one mount or another, stick with it though! you never know!