These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
In the first Alice in Spaceland, after all the hell you've been going through (especially if you're playing on Hardor Lunatic Mode), you'd be surprised that Zio's pathetically easy if you just get him to continuously circle you. The only thing that will be a considerable threat to you in the fight is a lone, respawning F.O.E. You can do the same thing in the final fight in 3, but the Zio following you has a weird way of reacting to your direction. That, and there's a second Zio attacking you constantly, so pulling the same trick will not go as smoothly.
The Nonoba Admins and Cirno in Alice Xi are also pathetically easy if you know where to camp at (in Cirno's case, you'll need to shift positions every time she changes her bullet patterns).
Goddamned Bats: Some of the enemies can verge on being this if you're careless.
That One Boss: Most of the games have a boss you will inevitably get stuck on.
Alice in Spaceland had two encounters with McGillicutty that were hard. One had a crapton of Asteroids enemy fighters firing at you like they were Space Invaders or something. Another had a three-on-one fight. Both were pretty hard, but if you're playing Hard Mode and beat the third encounter...get ready for the Combined Itanimulli Asteroid Fighter!
Alice in Spaceland 2 has Weegee. Yes, that Weegee. His hard to dodge projectiles can really be a bother, and if you're playing on Hard Mode...yep, he first goes Super Sayan (this happens on any mode), and then combines with Malleo (the Weegeefied version of Mario, who is a boss in Alice Xi) to form a 'hyper' version, which is much harder than the first form, and takes what feels like an eternity to kill.
Alice in Spaceland 3 starts off pretty lax, but once you hit 'The Crossroads', kiss a lot of your lives goodbye. Especially when you reach Havoc, a security system you will come to loathe and hate, you're bound to lose a ton of lives. Then he comes back in Cirno Xi, now at Mark II. And even worse, he comes back again as the regular Final Boss of Spaceland Adventure 2! Of course, he pales in comparison to McGillicutty. The fight before him was hard enough, but when you have a five-on-one fight and auto-aiming against you, all bets are off. Your lives are GOING to go down.
Oddly, Alice Xi is devoid of these. The bosses are considerably easier, and you won't really lose any lives unless you're not paying attention, not using a trick on some of them, or quick to react.
Spaceland Adventure actually isn't too hard, unless you're really careless at times. Like Alice Xi, it's devoid of these...until you get to Wave -1. Since your seven hit ship is destroyed thanks to Renko Usami, the other world Reimu, you now have to avoid and shoot blazingly fast enemies without being able to turn yourself now, and you've gotta avoid craptons of bullets, and then fight Ezam, the main antagonist of the Ezam series. All without being hit once. Thankfully, Renko is abosolutely easy thanks to the fact that, because she has been in Spaceland for a while, her bullets cannot, and will not, harm you.
Spaceland Adventure 2 has either Sinistar especially if he goes Super Sayan, or Havoc MK III. One moves ludicriously fast as the battle drags on, and the other...you should get the idea of his track record by just looking at his name.