Perhaps I was to quick to dismiss this I may have to give it a whirl
Here's my attempt at summarizing the pros and cons of Mounted Combatant:
(-) Logistics. You need a good way of procuring a mount in the first place, and/or replacing one if it gets killed by white dragon breath or something. Find Steed and Phantasmal Steed can both work, as can gold and downtime, but you'll need to plan this out. Also you need to feed it and care for it.
(-) Availability. Sometimes your mount won't fit into the adventure, e.g. if it takes place at the bottom of a well or at a duke's ball.
(+) Advantage on your melee attacks against Medium creatures: can cancel out the downside of GWM or simply increase your effectiveness against an armored foe.
(+) Mobility: free Dodges and Disengages on a high-speed (60'+) chassis means you lose fewer rounds of combat on closing to melee range. Or you can kite effectively with a longbow if you prefer to be more cataphract than Sir Lancelot.
(+) Extra opportunity attacks: attacks from your warhorse/gryphon/whatever make you stickier.
(+) Better blocking/tanking: a Large-sized mount can hold a chokepoint against a Huge giant. A Medium-sized human cannot--the giant can just ignore you and move through your space because it's two sizes larger. This makes you a more effective tank.
(+) Weird combos. If you have a mount that is capable of attacking on its own (i.e. an intelligent mount, possibly via Find Steed depending on DM ruling), you can sit up there on its back and Dodge every turn while the mount attacks. That gives you one attack at +6 for 2d6+4 (11) while also imposing disadvantage on any attackers. Goes well with the better tanking aspect.
(-) Stealth: Mounts are usually bad at it.
(-) Vulnerability to Pushing attacks: You can get knocked off your steed (possibly falling prone), and that will really disrupt your battleplan if you hadn't realized it was possible. Especially if it was a flying mount.