It's a pretty interesting mechanic that relies on luck. As such, I have some mixed feelings about it. On one hand, it makes vanguard more interesting, but makes it less consistent as well. Royal Flush relies on having you have less triggers in the deck, which makes up for the end game.
Royal Flush also changes the entire way of Playing Neo Nectar's Musketeers. More people will run Mirrka in their decks due to free shuffling ability.
With Royal Flush as it is now, I actually wouldn't be surprised if it Bushiroad would steal it if they come by this website.
Not to mention Legion pretty much counters EVERYTHING from the previous Seasons: 20k base attack (Season 1 & 2 used a lot of 10k base vanguards, and those that didn't: there are now better versions of their abilities), Early Game Attack (Season 2 & 3 Limit Break is extremely weak to early game attacks).
I tested Hyakki against Karitora and Yakahisa, he still stood a valid chance. Against Yakahisa because she was hit seven, Karitora because she had a weakness to him though... Though I'm guessing Royal Flush is not much to worry, it's the second ability that one needs to be weary of. If you know how to counter those abilities, you should be fine even with Season 1 cards (that aren't 9k).
So, Hyakki and Royal Flush pretty much even. Royal Flush's advantage against Limit Breakers is the use of powerful abilities from turn 3 if lucky enough. However, by Limit Break they begin losing out on such abilities if you're unlucky. I stated this on the Royal Flush, Limit Break supports this or this supports Limit Break. [That's until I release that support]