Every 15' of movement equals 1mph in a RM melee round.
Most fly spells do not allow for very quick movement. 75' a round is SLOW, especially for fighting, were very quick movemnts are called for. To fight normally, a PC is probably required to move at speeds equivalent to 100' a round, even if they declare no movemnt as activity. Dodging, parrying, avoiding other combatants, reflexive reactions. A fly spell would hinder combat in many ways, though far less if the PC can start and stop and hover. Most GM's I have met do not allow this, with circleing like an airplane the only option to "stay in one place".
So constant, fairly slow movement would make many movents impossible and predictable. Flipping over someone to get behind them is probably not likely when limited to 5mph. So is zooming straight up to get out of reach. There is no zooming at 5mph.
Another aspect is orientation. Keeping balance will be a major issue. Being struck will tend to knock a flyer off balance.
Both feet on the ground serves as a vital platform to push off from and generate force, something a flying PC at a slow 5mph will lack. The speed is to slow to replace the leverage gained from "toe to toe" combat.
So, easy fix is to demand an athletic skill: Flying. Any skill used while flying cannot exceed the flying skill. If your fly skill is 64, then your OB is 64. In addition, hits delivered are halved and max at St mod. All missile combat suffers an innate -30+Ag mod, while crossbows suffer no penalty but take at least three times to load (they may require landing actually). All melee combat is at -50+Ag mod.
Additionally, being struck while flying results in an additional unbalance crit, rolled seperately. Normally the crit will be one severity less than normal, with A crits resolved at -25. However, large attacks resolve crits equal to crit landed, and huge attacks resolve crit one severity higher.
When orientation checks are required, they are default hard, and don't forget the additional -20 for being in melee.
Most warrior types carry more than one weapon, and should always have something to throw, if not shoot. 75' a round is a very brisk walking pace, or a slow jog, so there is no reason for attacks against them to be that dificult.
When balance and orientation mnvs fail, the PC falls, at 32' per second per second. This means most will hit the ground in two to three seconds if they were staying in melee and spell casting range. The fall/crush table is one of the most powerful attack tables in RM, partly because falling is so very, very dangerous. Use the table.
Under no circumstance am I suggesting you be unfair or fudge to stop them. let the dice fall were they may. However, flying is dangerous and limiting your movent in combat is extra dangerous and fighting while flying certainly sounds very dificult to me. Finding trainers for air combat is likely impossible, so PC's won't be able to learn these skills until later in levels. That will slow them down, maybe even stop them. But...
They may decide to play nice, follow the rules and devolop the skills needed (flying, acrobatics, etc). That is ok. From the start here you should be stressing that melee combat is extremely dificult to perform while flying. Missile and spell casting is tactically superior. They should get the idea after you unviel your new rules for flying in melee.
