Seven and Celceta are probably two of my least-favourite Ys games, to be honest. I don't like the party system at all, and I feel like they're too verbose/drag on fairly long. Celceta does a much better job than Seven when it comes to balancing (Flash Guard is harder to pull off, thankfully) and swapping, though. I played the older versions of Ys IV years ago and liked them more.
Then again, my favourite games in the series are more along the lines of Origin, Ys III PCE, Felghana, and Ys II, so what I look for in Ys is different from what those two games offer.