CR is not very reliable in determining the Strength of a particular (group of) monster(s) vs. a party of every possible constellation.My only true beef with the system is the that the monsters are far too weak once the party hits 5th. I routinely throw CR 12-14 monsters at my 8th level party and they do just fine.
My secondary beef is the assumption of 6-8 encounter days...with no adjustment for less than that. Sorry but I can't imagine making that my default every game session....once in a while sure...but every single time?
The 6-8 encounter is for hack'n'slay style or normal style + noncombat encounter. Means: The party meets a NPC and with application of social skills they manage to coerce info (= one encounter) or: The party has to scale an obstacle and invests some of their resources, e.g. cast spiderclimb, attach rope, makes some athletics checks with the chance of falling damage (=another encounter)
Having all 6-8 encounters being pure combat encounters would overstress my time budget for that pillar. So I never do this, instead I scale up combat encounters and use less of them.