5E *is* a well balanced and well tested system, which is why deviating from it so dramatically is puzzling - why take something that is known to be balanced (on average - there are always exploits/over-powered builds) and arbitraily change it? I have played D&D from AD&D up to 3.5E, PF, skipped 4E (blech) and returned to D&D with 5E. Initally I disliked it, but it has grown on me. There are legit ways of making 5E encounters more challenging within the game system - you really don't need to break the monster manual to do this. Monsters are even arranged by CR - so you can easily figure out what is appropriate for the party. And they have access to all of this. And as pointed out earlier, many monster aleady have powerful abilties - you can always find sometime that suits the scene. The odd homebrew addition is fine of course.
One positive: goblins have been slightly weakened - although they still have too many incendiaries - and minotaurs have lost some of their power, including (as far as I could tell) their AoE jump that knocks people prone (well, they landed but nobody in my party was knocked down). So those are hopeful signs.
