The revamped monsters in ' are much more dangerous thanks to a mostly overlooked change. Almost all the monsters in the new, which will be released later this month, have received some kind of overhaul or change.
Some of these changes are relatively minor, such as a monster dealing a different type of damage with their main attack, while others have had their core abilities mechanically changed.
The goal of these changes is to allow monsters to provide an appropriate threat at an appropriate level. While most of the focus on these new monster statblocks is on new monster designs, new attacks, or even new types of monsters, many of the monsters in the have received a major buff that will make them much more dangerous. Many high CR creatures now add their proficiency bonus to their Initiative bonus, which means that they'll more likely attack near the top of the round instead of the middle or end of the round.
While not as dramatic as other changes, the higher Initiative bonuses will make combat a lot more challenging right at the start.
Under normal circumstances, both player characters and monsters use their Dexterity modifier as their Initiative bonus. However, as noted on Wargamer, many character classes and subclasses now grant players advantage when rolling for Initiative, which typically results in them going earlier in a round of combat.