They are very good combat and transport mounts. The males can attack, which allows them to gather thatch very efficiently. The females have no attack, but are a little faster, as there are no horns to weigh them down. There's also something satisfying about riding a moose. WHEEEEEEEEEE
To tame a Megaloceros, I would try to tame an Argentavis and carry it to somewhere warm. Assuming you have plenty of Mejoberries from any berry collector, or even by hand, or you could have Dimorphodon kibble you can begin taming. I would suggest using a bola, because these guys like to run. With a few tranquil arrows to their head they would be out quick. These guys don't usually need much narcotics. The megaloceros is amazing at collecting thatch, and they have a bleed effect, which is great for pvp and killing dinos.
My favorite cave mount. I have been breeding them for months, my 776 melee outputs as much damage as a wyvern bite at 500 melee, plus bleed. Put an MC or better saddle on one and it will wreck any cave. They absorb otherwise rider inflicted damage, they can jump to clear gaps, they're fast, they output massive damage, and can hit more than one target at once with their attack. What's not to love? (Low health and weight which also makes them painstaking in baby stage)
Surprisingly good fighters, my rank 10 Mega is taking down carnos and argies on death island, and escaping with barely a scratch. I suggest upgrading health stamina and speed to maximize survivability.