So far, I've finished Phantasia, and tried Vesperia, Symphonia and Berseria.
I don't think I've seen anyone mention yet how these games are all Action RPGs. You control one character, while the others follow a mixture of AI commands and occasional direct commands from you. It can be pretty overwhelming at first.
I felt that the combat system of Phantasia was much easier to get into, since it was on a 2D plane, much like a 2D fighter. You had fewer defensive options, and your only means of avoiding damage were to stay out of an enemy's attack range or land a hit before they could get off their attacks. However, it was also easier to keep your AI buddies safe, since all you had to do was stick yourself between them and the enemies.
Once the series went 3D, that tactic stopped working, and it became much harder to see what was happening on the grand scale. On the other hand, your allies are a lot less defenseless. Still, it left me feeling like I was somehow playing wrong, since I no longer had a tight impregnable formation going, and was just leaving my party members to fend for themselves.
I'm playing Tales of Berseria now. The apparent complexity of the combat system has my mind reeling. I managed to steamroll the first and second dungeons, including the bosses with what little experience I had, but so much of the combat still feels so foreign to me. I hope I'll understand it by the time the game starts demanding real expertise.