You can take feats from different magical classes at lvl 4, so a strict rule that one class can only cast their native spells and no others is at least not in BG3. So as a Light Domain cleric I can cast Hex suddenly or use Blast Wave. Why then should a hardworking mage not be able to learn some "normal" cleric spells from scrolls? I think the magical stuff which comes from the gods can be easily supplanted by direct energy from ... whatever (I think the gods use it too).
Personally I can understand the criticism and I share it to a big part. I would prefer a restriction to class spells, a reduction of scrolls and some severe limitations for being able to use scrolls. But I also don't like multi-classing and the level 4 magical feats, so it's kind of consistent.
I tried the magical feats and dumbed it, especially the use of Hex outside the Warlock seems wrong to me (ok, my decision was helped a bit by the magical marker for Hex hovering above your head, I hate such magical effects ...). I never gave a spell to the wizard which is not from his school, actually I only used a scroll for this once because I had forgotten to choose the spell at level-up. I also don't use scrolls in combat, it's luckily not necessary.
So as long as I don't see lots of enemy wizards with cleric spells, I can live with the house rule not to use a game feature I don't like.
To class balance, wizards don't need cleric spells to be good in my opinion. But they also don't get op with it, as there are some restrictions still valid. I still hear Gale silently crying during the nights sometimes because he never ever will get such a good aoe as my cleric, hehe.
