Oop sorry I missed this.I mean, I'm assuming we've all run successful 5e sessions using primarily the PHB material. Why would we need the power level delta in a homebrew book to be even MORE stringent than the PHB?
Running content with a wide power delta is a PITA. I need to be aware of the power profile of the moon druid, ward new players off Berserker Barbs & Assassin Rogues, plan for owl familiar scouts, jack the power of enemies way up for the peace cleric, yadda yadda. With the PHB I can at least google "moon druid strong" / "5e assassin weak" and get a dozen+ opinions on each class's power, what levels they're problematic at, how to make their abilities relevant in stories, etc. With 3rd-party & homebrew stuff, I don't have that luxury, and my time gets stretched thinner & thinner as the list of allowed brews grow.