My party is level 8 in Avernus and they are going to walk into a room thats 50' high and 100' wide (circular). 2 Erinyes that can polymorph into giant spiders, 2 phase spiders, a shadow demon, and a fomorian. The place is littered in webs that will halve the speed and the room is hot and gives them a level of exhaustion every 2 rounds on a failed DC12 Con Save. I don't really run easy fights haha. The 2 chain devils riding wyverns and dropping them from 200 feet after they were taking crush damage was a rough one, just some random encounter. By powerful I mean a fully charged party focusing on one thing is powerful. I ran tomb of annihilation. 5 level 10s almost killed a CR23 Lich AFTER fighting something that was a DC19 con save to not because exhausted every round and that thing had 250 hp of its own. Lich had power word kill, maze, all sorts of nasty shit. The way the map is though, he cant really move around. Thats crazy though. It why all bosses have escape, legendary, or minions because players will annihilate solos. That is the issue though is for every head you put onto the initiative tracker, that power gets more and more and more diffused. It should be a battle of skills, spells, and being clever. At a certain point a large number of individually mediocre enemies A. Isn't fun B. removes utility from a lot of abilities and spells as it just doesn't accomplish much. 100 skeleton? You thunderwave and kill 10. 90 left. Cool spell?
This is up to DMs and players as to what they enjoy, of course, but I have found over the years that a spell landing or an ability sticking having a marked effect on the field is a lot more gratifying.than saying "Hey I got Goblin 34 in hold person for like 1 round until I got hit, im out of spells now". People like seeing that impact, like having the spell slot that this forum and others have already hammered on needing to be more important (resting mechanic), so if Im going to use a spell it should be friggin important, I dont have a lot of those. Resting afterwards takes away that decision making but, realistically, people using spells should still be a cool thing and its a lot more toned down than what I am used to seeing from 5e casters.
Players dominate solo encounters because of action economy and when the DM allows them to reach those encounters with full resources. Playing the game as reccomended means whittling down player's resources with minor encounters forcing them to make hard choices not to face every encounter at full strength, otherwise yes they'll just wipe everything going nova. A single monster however powerful can't dish out the same amount of moves a group of 4 to 6 PCs can. That's why we give the BBEG minions. As for spells if a player wastes hold person on goblin #34 that's entirely his fault, if he uses that same hold person spell at the right moment against the right opponent (that pesky spellcaster NPC? That huge hill giant who's weak against wisdom saves?) then the spell is not wasted at all and allows the party to suceed.