I didn't even knew Royal Guard was a thing until after I finished my Normal playthrough. Made my Nightmare playthrough lodsa easier.
But yeah, the party system really needs some good ironing out.
There doesn't seem to be any difference between the SP skills, aside from the SP cost and the shape of the attack. You only use it for MORE DAMAGE when the boss is stunned and just in general. You can see the differences between SP skills in MoC, thankfully.
Nor does there seem to be any difference in how another party member plays. They all come down to mashing the attack button, dodging, and using SP skills. The only differences being is their attack type, assortiment of SP skills and their EX attack. I expected to have a different playstyle when wielding a hueg hammer or shooting arrows than I do when wielding a sword. But party members just end up being additional life bars in boss fights. You should at least be able to order other party members around so they won't get in your way. It's not like they do much on their own but attack every once in a while and get hit.
Royal Guarding does make things 2 ez, you can stand in a pool of lava and just not get hurt. Honestly, Royal Guarding (or Flashguarding) should have some kind of limit similar to the stun bars of enemies, so you can't spam it all day. You DO receive 2x damage when Royal Guarding at the wrong moment, but you'll get the hang of the tempo sooner or later. No jumping means no platforming (although doing so with a party would end up with Dragon Slayer-style shenanigans) and a less varied attack pattern for bosses. Instead you roll around (or Royal Guard) everyone's attacks on a horizontal plane.
The final boss deserves a special mention for forcing you to level up your other six party members evenly, because you have no incentive to switch between two characters with the same damage type, aside from their passive ability and/or depending if you like their character (Dogi 4 life). So you'll end up with 4 underleveled party members with underleveled skills and underleveled weapons for the final boss, and that is not any fun at all. The game wants 7 playable characters (because it's called Ys SEVEN hurr durr) regardless, but like I said before, giving them a different playstyle would've given players an incentive to change party members every once in a while.
There was also that boss who you were supposed to lose to, thanks to his overpowered attacks. YOU COULD BRING HIS HEALTH DOWN TO 50%, AND THEN HE'D SIMPLY STOP RECEIVING ANY DAMAGE AT ALL WHILE YOU'RE SPENDING ALL YOUR CONSUMABLES TRYING TO FIGURE OUT A WAY TO KILL HIM. DON'T EVER DO THAT SHIT WHEN YOU DEVELOP A GAME.
The story isn't all that special (you are the chosen one for the umpteenth time), although it has some tweests here and there to keep things fresh. It does however spend alot of time on it through dialogue and cutscenes, moreso than other Ys games.
You'll also be backtracking dungeons (as your fast travel waypoints have been conveniently wiped away!) later on. There's also the whole crafting system, so you can gather materials by killing stuff and gathering stuff, and purchase special weapons and armor. To me it mostly incentivizes grinding (which Falcom probably intended with the final boss in mind).
The music is still great as ever, but there isn't much of a variety in tempo between the battle songs, so most of them sound kind of similar. I'm sure if you've listened enough J-Rock that you'll hear certain patterns pop up.
The gameplay is pretty damn fast, it's probably faster than the Napishtim games but not as fast as the bump system games. Despite its faults, I still had alot of fun with Seven. While the obligatory dialogue will leave you bored when replaying Seven at higher difficulties, it's still just basic fun. Can't say much for MoC, but I heard it's a remake of Ys IV with the gameplay of Seven.