From what I've seen in 5E, the CR is an okay guide, but matters a lot less than DM tactics. Whenever I make an effort to play a monster the way I think they would fight (canny tactics from bandits, infuriating pest-fighting from goblins, single minded aggression by monsters and constructs, predator -prey tactics by animals, etc) I'm surprised at how hard the fights can get.
Ex; 5 Goblins standing in a clump, shooting arrows at the Fighter while mindlessly taking damage is going to be a relatively easy fight.
A Goblin ducking behind the mage, giving advantage to the second goblin who dropkicks him in the chest, opening an opportunity for the third goblin to hold the hapless wizard down while the fourth goblin smashes a flask of lamp oil over his face just as the fifth Gobbo comes running up with the torches has the possibility to get hairy, especially since the goblins know enough to Disengage and book it as soon as the plan meets any resistance.
Long story short, I look more on terrain and circumstance than simple CR when crafting encounters for my players. It's a better guide.