That's the interesting thing: I'm finding that in 5E, my intuition is off, and many encounters that I would expect to be "run away!" adventures actually turn out to be 50/50 situations when I actually game them out. Monsters are weaker than I expect, relative to the official guidelines.
The only way I know to make monsters strong enough to challenge PCs is to turn it into a war game, about using smart tactics and something better than "the allosaur pack charges you at full speed"--turn it into "the allosaur-riding enkidus charge you, hollering warcries and firing arrows from their longbows". Raw stats don't seem to cut it in 5E. The problem with that approach is that I'm still a pretty new DM, and I'm not sure yet when it is appropriate to roleplay enemies with tactical smarts. So far I've been erring on the side of dumb monsters but I think I'm about to unleash the allosaur cavalry next game...
The game can also be quite swingy based on a few factors. Luck, party composition vs foes, etc.
My findings are in general things that just attack are mostly ineffective for their CRs. Things that cast spells or have spell like abilities are a good challenge for their CRs. Also custom monsters I have created using the DMG guidelines feel like they hit a lot harder than a lot of the standard monsters in the MM.
The way my group operates however is they generally conserve resources as much as possible until they get to a big fight. They've struggled with "medium" encounters at times, and they have totally dominated deadly x 5 encounters at other times.
What can make a big difference as well is controlling when they short rest. This can sometimes be hard without railroading them, or having some sort of timeline that forces them to hurry. Characters can fight well above their CR when they can 'nova' with short rest resources.
All in all it's a by-product of bounded accuracy. Bounded accuracy can be great in some ways, but it also can mean that your players (with careful resource planning) can totally dominate things well above their CR. Of course this works both ways, throw 20 well fortified Orcs with bows at a high level group and see what happens.