Feats, multi-classing and magic items are OPTIONAL. So if your game includes them then you need to compensate in the challenge department or things get much easier. So if you are using typical encounter guidelines and your campaign uses the optional features then then you are presenting a game with easier challenges.
The point is however that they still should have playtested such optional rules and made them more in line with the basic game. It is poor design work to make broken multi-class combos and feats simply because they are "optional" and some tables won't use them. Especially given that feats are some of the only means of character customization for non-spellcasting classes.
That also doesn't address the fact that many higher level spells are also game breakingly powerful. Spells are not optional.