It may be forgiving, but even if going non optimal, the use of feats and m/c still leads to a party with homogenized abilities and stale gameplay.
This is a common issue in classless systems which people wrongly assume will lead to more unique characters. Instead, you end up with characters that play very similarly.
The strength of class based systems is in setting limits on what a character can do to promote party diversity.
5e does a great job of allowing choices in chargen with archetypes and races. Those keep party members varied while giving them choice. If you add in m/c and feats, characters start to define themselves by low level dipped abilities or cheesy feats rather than by their class.
The best evidence of this is this thread. A huge debate over a single feat. This feat defines martial characters in some ways more than the bulk of their class abilities. Those who count themselves amongst the best martial characters are those whose class abilities synergize best with this one feat. But, ultimately, all martial characters using this will play pretty similarly.
If you take the feat away, all these characters will end up relying much more on their class abilities and this results in more dynamic play.