To break this down to its bare essentials, melee has a couple of really pertinent downsides relative to shooting:
-It has an effective range of 0, so you have to endure shooting and get in close.
-Because you have to get in close, it's easier to block just with screening.
-Your enemy gets to hit you back, even if they'd already shot in their turn. Any time you're hurting the enemy, they're hurting you too.
In previous editions, the main upsides were:
-You get to attack on both your turn and the opponent's turn, so good combat units can do a lot of damage quickly.
-You're safe from shooting, so a good combat unit can inflict damage while mitigating it in return.
-Morale checks for the loser made it easier (relative to shooting) to break the enemy and either wipe them out or render them combat-ineffective.
On top of the increased lethality of shooting (including Overwatch), the above upsides have been mitigated in 8th- Fall Back negates the first two, and morale is easily ignored.
GW has specifically identified Overwatch, Fall Back, and morale as mechanics that are changing in 9th. It's possible that we might be going back to an older version of how things work. Throw in that recent stratagems (eg the new AdMech ones) are focused on durability as much as firepower, and the pendulum might be swinging back.
We'll need to actually see what they've done to those mechanics to draw any useful conclusions, IMO.
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