From keeping a very close eye on propensity to alpah my observation is that party capabilities are very volatile. A creature that is a nightmare for one party, can be trivialised by the group who have the caster with the right spell. That same foe might be lethal, or a walkover, depending on the encounter set up.
That said, some features of such fights
- They generally play out better in the middle ground of 2 to perhaps a dozen creatures. A lone creature can suffer tempo problems. More than a dozen can take a long time to play out (assuming they all have actual teeth in the combat).
- Generally a creature needs a way to attack more than one PC - to deplete multiple health bars at the same time.
- Generally it also needs a way to dump on one PC, either with an SoS or a lot of damage
- Varied creatures yield a fight far, far more interesting than all the same type of creature
- With cheaper creatures, you have to think about how they are equipped. Take a look at Orogs versus your basic Orc. Orogs have far better armor, which really counts. Those 200 guardsmen? If they all have longbows and split into small groups, the party are going to struggle far more than if they rush in with simple weapons. They need to be varied - some plate and shield, some heavy crossbows or longbows, remove a few dozen and add a caster or two.
- Decide how you want to play morale. The DMG has some guidance on that. It really matters in making bigger, longer fights more manageable. Think about why the foes are in this fight at all, and what they want from it.
- Effects that change the tempo equation are very swingy, like a succubus charm. If it comes off, then the party are one down and their foes are one up.
- Usually, the foe getting a surprise turn will inconvenience one or two characters, and make the fight hard. If the PCs get a surprise turn, if they can freely alpha that is the end of the fight.
The basic trouble with alpha though is your fights become all-alpha, all-the-time. The same most effective spells are cast again and again. The same abilities are always dumped into the fight.
Against that, trivial fights are also dull. The space for interesting is created by fights that are probably a bit fewer in number than DMG guidance, a bit harder on average than DMG guidance, occasionally deadly or deadly+, and your party can sometimes afford to alpha but you run rests in a way that forestalls that being every fight.