D&D 5E Downsides to Mounted Combat?

[Shiroiken]

Tricks are 100% more effective in a solo campaign than in a party campaign.

Just attack the guy not on the mount... as opposed to attack nothing because the mount guy flew out of your effective reach

True, but a low CR creature had little to no chance of hitting the Paladin if he didn't use Mounted Combat, making the complaint about this specific scenario irrelevant.

As for the "trick" of mounted combat, a creature with reach, ranged attack, or spellcasting could still ready to attack the paladin as soon as they are able. They arguably lose Multi-attack, but that's a reward for taking the feat. When using creatures lacking these things you shouldn't even roll out the encounter; the enemy can't affect the paladin, so they will either run or die. The paladin's 23 AC is a much bigger problem than mounted combat should be.

 

[Greenstone.Walker]

Indeed, Sage Advice is just repeating what the book says. And though you cannot use multiattack for an opportunity attack, you can for a readied action (by the book).

Are you thinking of Extra Attack? Multiattack is something monsters get, not PCs.

In any case, neither of them can be used with a Readied Action.

The Extra Attack feature states, "Beginning at 5th level, you can attack twice, instead of once, whenever you take the Attack action on your turn." Multiattack states, "A creature that can make multiple attacks on its turn has the Multiattack ability. "

Readied attacks and opportunity attacks happen on someone else's turn, not on your turn.