Its not a big deal, as long as you have the same number of spell slots... the cleric 11/wizard1 example is just a little better theurg, but its just marginally...
Yes, it is a big deal. By PnP rules that character would be able to cast 6th level cleric spells and 1st level wizard spells. The suspicion is in BG3 that character would be able to cast 6th level wizard spells. That's a massive power boost over PnP.
There is no "Mystic Theurge" in 5e.
