Has any edition of D&D ever done solos well? Lair actions help, and I generally up their power level. One really simple thing is to make number of legendary resistance/actions number of PCs - 1. That and throw in an environment that's advantageous to the bad guy on top of all that.
I think it's another topic though.
They never did, yet they all did. It was and is easy to get on the single boss, beat it and call it a day. Be it dragon, lich or even a god (in 1E and 2ed...). The real challenge of BBEG is the DM.
How did the players got to the BBEG?
How did it react?
What steps was taken by the BBEG to ensure the players would not get to him?
A lot of questions that are often answered by (in order)
Easily.
Did nothing.
Did nothing.
This reminds me of a DM in the '80s that told me that he could no longer challenge his players. They had killed Odin... I said what????? Impossible. He was so sure of himself that he talked his players of reenacting the battle with me as a DM. I utterly destroyed the group in less than 10 rounds... Players were tempted to say I cheated, but I rolled (and still rolls) everything in the open. They were less than happy. All this to say that difficulty is in the hands of the DM.
Whatever the game system. Be it Palladium, Role Master, Star Wars (WEG or D20), Call of Cthulhu, Paranoia, D&D (whatever the edition), Heroes, Battletech, War Hammer, Vampire the Masquerade and its offshoots or any other system that you can think of. The DM sets the difficulty by either enforcing a strict view of the rules (such as I) or by modifying them to his tastes. Is 5ed really so easy? I don't think so. I haven't had so many TPK in 3ed or 4ed. Both of my group, one with veterans and the other group have about 10 years of experience in RPG, find 5ed quite challenging and don't understand how people find this edition to be a Carebear edition. We were at the game store a few weeks ago and there was a game going on. I was shocked to see the DM allowing so many things that were not in the rules or blatant by passes of rules. And yet, he was complaining that 5ed was too easy (so were his players). These guys were allowed an ASI every four levels and were also having a feat alongside it. So that was double ASI or feat in any games that I know of. It was done in the name of "customization". There was additional feats found on the net that had also been added. But you can see where this led. Games were only one or two encounters per day with easy rest rule. Nova was rampant in this game. I gave them a copy of my .DOCX of: "What you can do in a round of combat." A DOCX that I give to every new players that come my way. I did not have any news so far but the shop owner took my .DOCX and he printed out a few copies to give to people. I think it helped people in that area.
As I said, 5ed lacks a lot of clarification about what and how it is expected to be played. Unless the DM is experienced, you might see strange things going on. I don't know everything and, thankfully, be proven wrong on some rules of 5ed on this forum. (THANKS A LOT GUYS!). Other times I was right (Thank god!). But it is by coming on this forum and by talking with other DM and players alike that we can better grasp the DOs and DON'T of this edition.