4E had similar problems in the first couple of years. High-level monsters were pathetic for their level, and much too defensively oriented. It wasn't till MM3, two years after release, that they got things ironed out. I'm hoping they'll get a better handle on high-level monsters soon. (It'd be nice if they could go back and errata the CRs of existing monsters, too.)This is why I quit 3rd/Pathfinder, CRs were meaningless, you had to do way too much work to run a game and anything high-level was just completely unbalanced. I'm considering dropping 5th after this and going back to running 4E.
That said, you can't expect the system to know how skilled your players are. If you have a bunch of optimizer/tacticians, and you're throwing standard-difficulty encounters at them, they will stomp all over them, because the standard difficulty is geared to the standard player. In 4E, the standard encounter is supposed to be at the party level (plus or minus 1), but I quickly learned not to bother with anything less than level+1; with my players, that was the minimum for an even mildly interesting fight. Level+2 was my go-to encounter difficulty, with level+3 for tough fights and level+4 for when I wanted to make them really sweat. And my group was only moderately optimized.