Is Treantmonk right or wrong about Mounted Combat?

[diplomancer]

My Dwarf cavalier took the Mounted Combattant feat at level 4, which he often used whilst riding the Bear-shaped Druid before he bought a horse.
My Gnome Beastmaster just bought Mithral Armor Splint for her Giant Badger.
My Wizard just picked up Phantom Steed after the death of her horse, and she's roleplaying her guilt.

The other are contemplating buying Barding armor for their mounts, although I'd expect the Paladin will probably learn Find Steed soon.

However, I DON'T believe attacking the Mounts is always more logical for the monsters. At their level (5) killing their Riding Horses often require an average of, what, 2.3 actions for the monsters ? If it's not killed round 1, having the mount Disengage pushes that closer to 3 actions.

From the monsters point of view (I'm assuming tactical hobgoblins here, we're playing Red Hand of Doom after Lost Mine of Phandelver, so encountering SOME hobgoblins outdoor IS a typical encounter), killing the mounts will cost the PCs, but it won't WIN THEM THE FIGHT. It makes sense if the fight is one sided : to prevent the PCs fleeing or pursuing. But in a tight encounter, sacrificing 2 rounds of actions so the Dragonborn Paladin is less mobile is a risk. My hobgoblin, knowing he's one hit from death, might rather use a Hail Mary and attack the Paladin itself, hoping for a critical Hit.

From the player point of view, the occasional cost of replacing the mount is the same as a Health Potion, and if the enemy spent two attacks on the horse, He probably saved you as much HP that the potion would have healed (you have to adjust the HP to take into account the horse lower AC).

(And, (because I'm generous, and because the players LIKE their mounts) I allow death saves on horses. )

It does happen, of course, and yeah, Area of Effect are a hard counter to horses, but not many foes have AoE in tier 1 and 2, IME. My players remember painfully the ones who did.

Well, in regular 5e, from the Monster's point of view, running away and trying to bring more allies into the fight is almost always the best tactic. But you can't run away on foot from mounted enemies, so you have to try and kill the horses to be even able to do so.

It also looks like that you don't require Animal Handling checks to bring untrained Riding Horses into battle. That is a big enough boost to cheap mounted combat that ruling separate turns might be necessary for balance. If you are talking Warhorse AND Barding, that's a BIG cost; might be worth it, even with separate turns, in a campaign where buying magic items is not an option once money stops being an issue... but then that's connected with a separate problem of 5e...