The difference with YoKai Watch is that there's RNG (and unlike Eureka, only the FATE's had a chance of dropping a token. So you'd be killing X amount of mobs, or a boss, or some other task the FATE has and probably not get anything out of it. In Eureka, everything has a chance to drop crystals). In Eureka, you can get your weapon AND gear faster than the Atma Weapon and glowing YoKai mount.
I can't speak for those forcing themselves to 20, as that's not what I'm doing. I'm taking things at my own pace (not doing it on the weekend) and I'm sure people will still be doing Eureka tomorrow.
It is an oversight that SE didn't include Level Sync, but you only really need level sync if you plan on grinding mobs and not being a part of the "NM train."