I think battle regimens were vastly easier to execute and understand than skillchains in FFXI.

The two main things that make them superior are that they're in a separate mode, and that there's a list showing what will execute. The separate mode means you're not telling the tank or other members not involved to hold off so that it can be performed - they can do whatever they want because they're outside the BR system. The list means you don't have to write a bunch of macros and try and time things perfectly with other players; good timing makes things smoother, but you aren't relying on people to notice things, exchange TP information, etc. because you can just see what's in the list on your screen. The problems of FFXI-style SCs would be made many times worse by the battle speed in XIV too.

One suggestion I have for making it better, though, would be to sort of combine both systems. Maybe instead of entering BR mode, you could get a list of recently used spells and TP attacks from everyone in the party. Then if you have something that can create an effect with it, you pick theirs from the list, then execute yours. That would retain the on-screen information and negate interference from other party members just like BR, but allow you to execute things on the fly as well as not keeping you "stuck" in the BR queue waiting on other players.