Making units act concurrently doesn't complicate things, it simplifies things: a huge array of strategic options are no longer feasible.
- Initiative is meaningless in real time (which is extremely important in D&D-type systems, as anyone who's ever played a D&D caster will know)
- Movement ranges are meaningless. There's no reason for a unit to stop moving before it gets to its destination, and there's usually nothing its opponents can do to stop it before it actually gets there (especially when there's several units moving at the same time)
- Movement is less precise (you lose 5ft steps, running, withdrawing, etc; anyone can move any distance at any time)
- Attacking is less precise (you lose the ability to strategically use single attacks, full attacks, keeping opponents out of range with bull rushes, etc)
- You lose "Ready vs." actions, because everything happens too fast for them to be useful
- You lose reach weapons, because anyone can just waltz in and out of your threat range whenever they want (again, because everything happens too fast)
- You lose the ability to aim spells effectively because units keep moving all over the place, making it extremely difficult to avoid friendly fire (every time you move your tank, enemies just immediately follow him; you never get a chance to throw a spell after he backs away, nor do you get to open combat with a spell as enemies often are right next to you by the time it goes off)
- You lose the ability to use positioning to control the battle effectively (i.e. you can't use Enlarge Person on your tank to block a hallway, because it's not tile-based, so anyone can walk wherever they want, whenever they want)
- You break the action economy (no swift actions, quickened spells, etc.)
- You lose surprise rounds (even if you notice the enemy first, by the time whatever attack you're using goes off, they'll already have started attacking you back)
- Attacks of opportunity are a lot less useful. Making a Lockdown build is almost out of the question.
The reason it's hard to do these things in real-time is because this system is not meant to work in real-time. It's just a bad combination. The strength of a real-time system is quick, twitch-based gameplay (Star Ocean, Diablo, Dark Souls, Witcher, etc.). It's very very bad for strategy-RPG type gameplay.
Play ToEE, then play NWN and compare. There is a huge difference.