There may be a lot of loot, but the mounts are still not guaranteed by any means. Blizz admitted the actual scenario is more like they're 5% drops at best. Out of the mythic raids I've seen:
O/114 Sylvs
0/103 Jailers
5/99 Nines
.. which is rapidly getting to statistically broken levels (aka the buff is likely broken for some mount drops),
Your mileage may vary, as always with RNG.
I went into the Collectors' Bounty event needing only 8 mounts from the dungeons and raids pre-DF, and I got them all within 2 weeks.
G.M.O.D. off High King Mekkatorque : I ran Battle for Dazar Alor 100+ times in LFR (I have 60 level 80 characters thanks to all the crazy XP events plus the insane XP post-expansion stuff they did after DF) and it never dropped, so I think LFR is bugged for that one, but it dropped after only 4 normal runs.
Glacial Tidestorm off Jaina in Battle for Dazar Alor (Mythic only), dropped after 4 runs or so.
Ny'alotha Allseer off N'zoth in Ny'alotha (Mythic only) : dropped after around 20-30 runs across two weeks, but I saved time by having the shortcut unlocked accountwide for that raid.
Vengeance off Sylvanas Windrunner in Sanctum of Domination (Mythic only) : got very lucky, got it the second time I ran it during the event.
Sanctum Gloomcharger off The Nine in Sanctum of Domination (any difficulty), got it quite fast too, maybe 10 runs or so, I think it dropped on Heroic but it should be fine on any difficulty.
Zereth Overseer off Jailer in Sepulcher of the First Ones (Mythic only) : got quite lucky, it dropped after about 5 runs or so.
I was missing 3 dungeon mounts : Underrot Crawg (mythic only from last boss in Underrot, it took me around 70 kills across my alt army, but you can ride through the dungeon and skip all bosses so it takes like 3 minutes tops), Tomb Stalker (mythic only from the last boss in King's Rest, dropped after only 2 runs), and Cartel Master's Glider (off the last boss in Taz'avesh, can drop on Heroic and can be farmed endlessly because this dungeon has no heroic daily lockout, but bring a monk to death touch on last boss otherwise it takes forever, that one dropped after about 20-25 runs).
I'm now just going to do the few raids where I'm still missing some transmogs since the event also doubles the amount of loot dropped by bosses. All I can say is good luck if you're trying to collect a mount you want, and keep at it, most of my friends in game also got a few mounts already.
Realistically, most people don't have as many high-level characters as I do, though Blizzard has provided so many crazy XP events that it's very easy to level up a bunch, so there's a good chance you're not going to get some of the mounts you covet if you only have 1-2 characters running the weekly stuff for 4 weeks, but the boost is definitely working nicely as far as I've seen (except for LFR G.M.O.D. as I said, pretty sure that one's fucked because it doesn't drop off the same boss on LFR and some intern probably missed that).