I've seen 5e sessions be cakewalks, and I've seen them be absolutely brutal. There's a lot of factors involved, including, but not limited to:
Group makeup. All Fighters? All Clerics? One of these games is not going to be like the other.
Teamwork or lack thereof. Built your character to be an island, neither needing or wanting assistance, nor caring about your teammates? You're going to end up losing allies, which means suddenly you best be able to solo encounters!
Optimization: playing a Monk with a 14 Dex because it suits the character? Congratulations, you'll probably have it rough, but you'll get by. Playing a Gloomstalker Ranger multiclass with optimized stats? Things may go a lot more smoothly for you, depending on your tactics.
Encounter Guidelines: CR is far from an exact science. Two creatures of the same CR can be worlds apart in the way they actually challenge a party. And a mob of low CR creatures can be far more dangerous than one big, jumped up Legendary beast! I once made mincemeat of a group of level 3 characters with giant rats, lol (3/10, do not recommend. A group of critters with Pack Tactics is a miserable experience on both sides of the table).
Player Experience: this ties into teamwork a little, but knowing to focus fire, when to use abilities and when not, and being able to quickly identify large threats can be the difference between a walk in the park and a Kobayashi Maru.