I'm one of at least tens of thousands who have used Mixer because the Smite Pro League signed a deal. After over a year of watching as much SPL as I could, I have literally never used Mixer for anything else and here's why:
Stream quality is a constant battle: I can have Twitch open and playing 1080p 60fps with a YouTube video also streaming 1080p 60fps while ALSO downloading my steam updates. If I want to watch a Mixer stream I have to close almost everything and if I'm lucky it won't buffer too much at 1080p, but normally I have to settle for 720p.
VOD playback: Again trying to stream a VOD at 1080p requires preparation, it shouldn't. I should not need to leave a 4 hour VOD open for an hour before watching in order to get a consistent stream.
VOD availability: I missed this year's world championships due to work, I was halfway through catching up when I realised the VODs have been deleted. I believe there is an agreement between Mixer and Smite to not host these videos elsewhere, this means I no longer have access to these videos, and possibly more importantly, I now have a grudge against this platform.
Now I'm most likely not the only person to feel this way, meaning you potentially created a grudge with thousands of people, Smite is now going back to Twitch and I'm likely to delete my Mixer account and never look back. Overall, my advice is do not try to get an exclusivity deal when your service has almost zero advantages and several disadvantages compared to the already existing service.