I find this entire "overpowered" idea to be potential "lazy DMing".
Monsters have many of the same synergies that PCs have. The difference is that each player is optimizing his PC for the long term. Game after game, encounter after encounter. The DM has disposable monsters, so he might not be optimizing his monsters or their tactics hardly at all. On top of this, many PC classes/subclasses have spells. Most monsters do not. Spells are like guns in the real world. They are great equalizers. Players can start pulling out spells that can seriously hinder their foes or help their allies. This is one of the reasons why PCs might seem overpowered. They have go to the well abilities like spells and can change the course of the entire battle in a single turn.
To give an example of how a DM can up the monster tactics a bit, in our next game, the 6 5th level PCs and 2 3rd level NPC henchmen (for lack of a better word) in our group are about to take on 11 normal Orcs, an Orc Warchief, and a female Orc Shaman (basically Druid from MM). They are outnumbered 13 to 8, but the Orcs are HD 2, the Chief 11, and the Shaman 5 (but only 4 in casting level). So, 2650 XP or a medium challenge rating (note: the adjusted XP rules in the MM would have a DM multiply this by 3, obviously, this is not a 7950 xp deadly encounter by any stretch of the imagination, those rules are awful). And, there are some extenuating circumstances at the moment:
1) The party was previously split up and lost their equipment via a magical trap, so the PCs do not have their equipment. They are using borrowed equipment from other PCs (i.e. the Cleric has mostly the Fighter's stuff, the Fighter has the Rogue's stuff and the Ranger's stuff, etc.). So, some PCs have better AC than normal, some have worse, some have magic weapons they never had before, some lost their magic weapons, etc. A few cannot cast some of their spells because they are lacking specific material components.
2) Some of the party found each other, so the party is now effectively split into three groups. One group is heading towards the Orcs from the west, one from the south, and one PC is invisible in the middle of the large Orc room.
3) The Orcs already encountered some of the PCs. Three Orcs chased after the PCs and got killed, one Orc went back and warned the others and woke up the Orcs that were sleeping. So not only are the Orcs alert and ready for danger, they have also knocked over some tables and beds and are using them for cover as they prepare for a fight with bows and javelins (note: some of the Orcs have longbows, I find it dumb that all monsters use wimpy ranged weapons).
The rooms adjacent to this big room are well lit, so no chance of surprise by either side.
According to the normal guidelines, this is a medium encounter. But, that does not mean that the DM has to play the monsters like idiots. The Orcs have been trapped in this location for some time. They know the surrounding rooms well. They would have set up some traps given the chance, but they just do not have the materials or expertise or terrain to do this. So their leader (who is crafty) has devised combat tactics for when they get attacked (and this has happened due to "wandering monsters", the PCs are in the Ruins of Undermountain).
Tactic 1: Several of the Orcs are ordered to become cannon fodder. One Orc rushes up to a one foe and Dodges instead of attacking. Another Orc does the same. Another, the same. One Orc per enemy. This greatly increases the defense of that Orc. If the target disengages, he lost an action. If the target attacks the Orc, he's at disadvantage. If the target uses a ranged attack against anyone else, he's at disadvantage. This does nothing against spell casting save spells, but it does weaken a lot of other options. The Orcs not dodging focus fire ranged attacks against one or two other enemies. This tactic limits the mobility of the PCs and blunts the effectiveness of many of their attacks.
Tactic 2: The Shaman will be ordered is to seal off a section of the room with the most foes with an Entangle spell so that the Orcs can use Tactic 1 on the remaining enemies. This tactic might be used again later on in the fight once the PCs start dropping Orcs and most PCs are out of the initial Entangle, but it could also be used against specific foes if the Chief considers certain PCs to be major threats.
Tactic 3: Heal the Orcs. The Monster Manual does not give the Druid NPC Healing Word, but I am. It's stupid for evil spell casting creatures to not have heal spells from their deities. This will mostly be used for the Shaman herself, but if a tactically well placed Orc is a little wounded, she might up his odds of holding.
Tactic 4: Fight the wimps. The Chief is going to attack PCs in lesser armor. The faster he can drop PCs, the faster his Orcs win the fight.
Tactic 5: Swarm. Orcs have the aggressive feature, so their mobility is much greater than PCs. The leader will order 4 or 5 of them to swarm a single PC and overwhelm him.
Tactic 6: Stay behind cover and use ranged attacks. Their ranged attacks do not do as much damage as their melee attacks, but the more the Orcs do not get hit, the more they outnumber the PCs in attacks per round (assuming tactic #1 is not in use). This tactic is probably a last ditch effort type of thing. Tactics 1 and 5 will find a lot more use.
The PCs are supposed to win most fights, or if too challenging, run away from them. A DM who takes time out crafting his encounters will find out that the PCs do not seem as overpowered as he first thought, instead, some of them might seem underpowered.
If the PCs seem to bust through encounters right and left too easily, it's really not that hard to either up the difficulty of the encounters, or add some decent monster tactics / terrain. But, it takes time and effort ahead of time for a DM to prepare his encounters with forethought. Encounters should not just be easy or medium. Throw some hard and deadly encounters in the mix.
I feel that too many easy encounters are a waste of time. It doesn't make the players feel empowered, it just blows through resources. Meh. Challenge your players and they will not seem overpowered.