I have run 3 campaigns at high level now. I can answer questions but I have some biases. My players start with arrays better than the default. I give out a couple feats and tertiary ability boosts over the course of a campaign as alternative rewards. I make custom magic items that do not affect attack rolls, AC, or saves for my players. I preach ideas like "You're not a tank if you don't threaten the enemies enough in combat to be a priority target," "preventing damage is better than trying to heal it after the hits," and "controlling the entire horde of enemies is better than fireball unless your fireball actually kills most of them."
An orc horde is sort of viable unless the party is built for a lot of area spells. PC AC doesn't get out of control unless you start allowing a lot of +X armor/shields/accessories, so the orcs always have a chance to hit greater than 5%, especially if a crafty DM remembers the Help action to grant advantage.
Epic and superheroic level play requires some homebrewing and DM cleverness. High CR creatures as written are pillow-fisted sandbags relative to PCs because the writers gave them above-CR HP and non-AC defenses coupled with below-CR DPR. A few exceptions like a lich exist that are very deadly.
My groups have consistently crushed high CR creatures with their offensive capabilities. I have scaled back the numerical effects of magic items significantly because of the impact they have on effective CR as I have adjusted over 3 campaigns.
Solo encounters at high levels do not hold up at all unless they are custom creatures with legendary actions and lair actions.
Your mileage may vary if you play with a bunch of blaster casters, healers, and "tank" melee. The bar is set low so newbie ideas work, and a DM can adjust upward as necessary.
