Actually, transport of troops is exactly the same as transport of goods. Warships were oared, yes. Troop
ships were typically not, especially since in most cases troop transports were merely requisitioned merchant or cargo vessels - which in most cases relied solely on wind propulsion, as rowers were too expensive. Byzantine navy did utilize modified dromonds for troop and
horse transport, though.
And relative importance of sailed vs oared ships is
much more complex matter than that. In fact, advances in sailing
technology and technique as well as hull construction meant that by 13th century sailed warships were preferred over galleys in both Baltic and Mediterranean. By 15th century Genoese were building carracks up to 900 tons of displacement. But in 16th century introduction of bronze shipborne artillery meant that galley were again preferred due to greater maneuverability and lower silhouette - cannons were few in number, placed on main deck and typically in chase arrangement, so galley could fire against hull of a sailling ship while shots from latter would sail over the galley. From 1520. to 1580. galleys dominated even the North Sea. In 17th century, introduction of gun ports and cast-iron cannons meant that sailing ships gradually replaced galleys.
But main issue here is distance. Usage of galleys meant that navy was merely amphibious arm of ground power. Venice conquered Croatian coast because they required
harbours for rest and replenishment, as their galleys could not otherwise make trips to East. In other words, galleys follow coasts. It is a supply issue - Byzantine dromon could last
no more than 7 to 10 days on sea.
Numenor is some 20 days away, which is twice the endurance of oared warship. So
Numenoreans either used primarily sailing ships for troop transport, or galleys supported logistically by sailling ships.
Scythians were defending from invasion in their own home area. It had nothing to do with "sluggish infantry" - they generally avoided combat - and everything to do with logistics. Huns and Mongols both made large use of local settled
peoples. Mongols came to Europe with Chinese engineers and siegeworkers. They were also nomadic peoples, which means that they basically give big F.U. to logistics of settled
armies (same as Scythians). But even so, Mongol armies which came to Europe were not that large, and when Europeans actually got their crap together, they cleaned the clock with Mongols. Second Mongol invasion of Hungary was amazing failure (but naturally it is the first one people remember, and thus you get the "invincible Mongols" trope...).
Basically, pack
animals pulling heavy weights (this includes war
horses as well, much larger than steppe
ponies) cannot rely solely on grazing to satisfy their caloric intake. Horse or oxen pulling wagon with
food will eat the same food wagon is carrying (or food carried in one of other wagons). Wagons are pulled by horses, so increasing number or size of wagons will also increase number of horses. At one point - cca 300-odd kilometers according to my calculations - you get to the point that animal (mule in this case, IIRC), needs to eat nearly all food just to get itself and the wagon to the end point. Grazing can increase this distance, but not indefinitely, and at the expense of significant time.
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