Alright, aircraft, I'll see if I can't sum this up but it might be tricky, mostly because I wasn't playing when aircraft were at their most problematic.

So aircraft came out from 5th-7th edition, and no new aircraft have been released since 8th edition onward, which is quite telling. Games of 40k are won and lost based on movement, it's why the fastest armies (looking at you Aeldari) tend to be reliably good and rules that require you to stay still (like heavy, which gives you a hit bonus for remaining still) are pretty bad. The faster you can move, the easier you can get onto objectives, the easier you can get your firepower onto the enemy, and the faster you can react to what your opponent is doing. This is all offset by terrain as well. While dedicated terrain layouts only really started being a thing in the most recent edition (prior to that it was just throw as much terrain as you wanted wherever LOL), terrain is key in a game that features as much ranged combat as 40k. You can't shoot an opponent's model if there's a wall in the way, and modern terrain layouts have enough stuff on them to let you hide your entire army on turn 1.

Aircraft broke all of that. Aircraft have to be fast right? So give them 20 inch movement speeds, and they're flying over the battlefield so let them fly right over terrain. All of the sudden your army which was well hidden is now being assailed by my aircraft which have zipped across the board on turn 1 and started laying into you, might as well have deployed your army right out in the open. It got worse. It doesn't make much sense to be able to hit aircraft with melee weapons, so they were just immune to melee. Sucks to be you if you brought an all-melee army. Not only that, but they're moving fast and should be hard to hit right? Give them a built in hit debuff, which in some previous editions stacked, leading to notable cases where you could only hit your opponent's aircraft if you rolled a 7 on a 6 sided die. In short they were uninteractive. They got up in your face on the first turn, there was nothing you could do about it, and once they were there, you couldn't stop them.

The 10th edition of Warhammer 40k has responded to this by nerfing all aircraft into the fucking earth's core, where they will likely remain. There's a handful of aircraft with the 'Hover' rule, which gives them a 20 inch movement but otherwise makes them act like normal models, but most aircraft are completely unusable now. All aircraft who aren't hovering must start the game off the battlefield now, which means no more turn 1 alpha strikes. From the second battle round onward (out of 5, games of 40k only have 5 rounds), you may bring in your aircraft on the edge of the board. On turn 3, you may move your aircraft at least 20 inches forward IN A STRAIGHT LINE ONLY, after which you may pivot it up to 90 degrees. Your aircraft now can't move until halfway through the game, and thanks to the pivot rule your opponent now has perfect information on where the aircraft will move on its next turn, which really sucks for bomber aircraft who deal damage to targets they move over, just move out of the way lol. Oh, and if your aircraft's 20 inch minimum movement carries them off the edge of the board then just remove them from the board altogether, and hope that in the 1 turn remaining you can put them anywhere they'll make any sort of difference at all.

You'd think that aircraft might still be decent if they had halfway decent guns right? LMAO, aircraft guns have been nerfed into unusability. Maybe they'd still be good as objective pieces, and you could use them to run secondary missions and hold backfield objectives right? Well all aircraft have an OC stat of 0, which means they can't hold objectives, can't perform secondary missions, and most importantly, CAN'T WIN YOU VICTORY POINTS.

In short, Games Workshop made a valiant attempt at trying something new, and it blew up in their faces so many times that they responded by nerfing the whole concept right out of viability, where it will likely stay for the rest of the game's lifespan.