https://store.steampowered.com/app/3212510/Encis_Solution_Demo/

It had a low, stuttering framerate on my 1060 6GB, but the level design and controls were pretty good.

https://store.steampowered.com/app/1521360/MainFrames?snr=1_25_4__318

Some gimmicks were unintuitive, like the optional hostages moving/interacting with objects in ways you wouldn't expect, but it's still fun overall.

https://store.steampowered.com/app/2886950/Duckthings_Rolling_Rooms?snr=1_25_4__318

The demo has quite a few tricky puzzles without ever doing anything unintuitive, which is always nice to see.

https://store.steampowered.com/app/3010840/Alpha_and_Iota/

Although there's quite a bit of platforming, the game's main movement mechanic is more top-down-like; you can move in any direction but have to reach the next node within half a second or so, or else you'll be kicked back to the previous node. It's fairly unique and the demo does a good job building on this premise.

https://store.steampowered.com/app/2944340/Unless/

Tight controls and solid level design, but unfortunately, one level can only be unlocked by beating previous levels fast enough. Hopefully, those levels will all be optional instead of the full game doing any Dustforce/Neon White nonsense…

https://store.steampowered.com/app/2597180/Whirlwind_Magician?snr=1_25_4__318

The worst thing this demo does is recommend that you use keyboard instead of controller. Ignore that, and you've got a promising start for a challenging-but-fair metroidvania.

https://store.steampowered.com/app/3094270/Crash_Puzzle_HammerSan?snr=1_25_4__318

Pretty good Solomon's Key clone. The tutorial does a decent job running you through the game's mechanics, and while it can be annoying having to restart a level for something you didn't plan for, there are no surprises: you can always see the entire level at once, and enemies/obstacles always act predictably, so it isn't a huge ordeal to figure out where you went wrong. It doesn't have a smooth difficulty curve, though; not even in the demo.

https://store.steampowered.com/app/1865960/Symphonia?snr=1_25_4__318

The demo for this remake has different level design from the original, free game, but is disappointingly way shorter. Still worth playing, though; only real issues are you can't go back for collectibles you missed and holding the Y button to open doors takes more than long enough to break the game's flow each time.

https://store.steampowered.com/app/2772080/Crystal_Breaker?snr=1_25_4__318

Solid SHMUP. Only issue I had was the demo didn't save my progress when I reached level 2 and turned it off, but that also happened with another demo I played (one that I know does save), so that problem might be on my end.

https://store.steampowered.com/app/2953590/CLONY?snr=1_25_4__318

The demo only had six levels, and four of them were pretty easy, but 1-3 and 1-6 tripped me up for a day or so. Still might wanna wait for a sale, though.

https://store.steampowered.com/app/3098730/Not_Easy/

Kinda easy, and there's not enough content in the demo to know if the full game is worth buying, but the free demo on its own is okay. It reminds me of those old Adobe Flash games.

https://store.steampowered.com/app/2959010/Lovish/

Okay controls, but levels have a surprising amount of backtracking for being only one screen large. It's also annoying when a hidden item shows up in a spot you can no longer reach because you had no idea the game expected you to plan for getting back there, but at least they're optional.

https://store.steampowered.com/app/3087900/Heritage__A_Dragons_Tale/

The demo waits to make you use the dash move long after you've forgotten you have it, and I swear weapon and enemy hitboxes are biased AGAINST the player, but despite this, level design is generally pretty good and the demo itself has around 2-3 hours of content, so it's worth playing on its own even if you ultimately decide not to buy the game.

https://store.steampowered.com/app/2336210/Monochrome_Heights/

This demo also has a solid 2.5 hours of content, but the entire first world (around three or four levels) has no hazards whatsoever, so the entire challenge there is based around making long jumps with controls just slippery enough that you'll only have a split second not to overshoot them. If you can make it past that, though, the demo gets much better, but there are still two levels where you have to wait for asyncronous moving hazards.

https://store.steampowered.com/app/1823030/Cursed_Broom/

The flight controls take some getting-used-to, and there's at least one optional page item that you can't get until you 1) bypass it, 2) get the power-up shortly afterward, then 3) backtrack for it. However, if you can get around that, it's not a bad little demo.

https://store.steampowered.com/app/3273110/TetherGeist_Demo/

Following in the footsteps of Celeste, this game won't let you replay individual segments of its long levels to get any optional collectibles you missed, and each world has a new gimmick that doesn't really affect the difficulty curve. However, the game also mimics Celeste's decent controls and challenging level design, so the demo is still worth checking out. I do think the jellyfish launchers need some work before the full game releases, though; they're kinda finicky. (that said, the demo got an update after I played it, so maybe they made things better already?)

https://store.steampowered.com/app/2976800/VARYZNEX/

Three of the game's five stages are in the demo, and it has some decent level design. That said, I never saw my health bar and never died (even though I got hit quite a few times), and the game's "blur" effect is really bad, but the demo is still worth trying for yourself.

https://store.steampowered.com/app/2954360/Ad_SHOOt/

Level design is okay for a bullet hell, though I'm not sure if the game lets you continue after losing all your lives since I finished the demo without getting Game Over (I hate games that erase too much of your progress on failure). It does do one thing I haven't seen other SHMUPs do: when you beat a boss, you get a new passive power that activates when you're not shooting, including filling a bar that gives you an extra life, so even if it ultimately doesn't let you save/continue, it's still more beginner-friendly than other bullet hells of its ilk.

https://store.steampowered.com/app/3246410/Kamikaze_Empire/

Very easy because you have seemingly infinite lives and dying doesn't reset the stage, so you can just keep throwing yourself at enemies and still win, but I had some fun with it.