If some spells are allowing STs at the start of turns, this is a very important bug that needs to be fixed. This makes some spells (hold person, etc) completely useless.
Compared to previous editions, casting has been made easier/less risky. There are no longer spells that take full rounds to cast, where taking damage during that time you can lose the spell. And casting a spell in melee range doesn't provoke an AoO, also possibly causing you to lose the spell.
The tradeoff is concentration; however this has been further heavily nerfed in BG3. Going prone instantly loses concentration, and any type of spash (fire surface) damage causes you to make a CONC save. But yeah, a lot of decisions in 5e spellcaster builds come down to which CONC spells you plan on typically using, and then choosing non-conc spells to complement those.
