As long as it's optional, I think it's great. It lets me play smash bros with friends who do tournaments. It lets me play, too, with literal children who are gonna have a hard time doing much of anything.

I think accessibility options in games are fantastic and as long as they're optional you can do no wrong.

I think the best thing, that's still not as common yet, is the ability to custom map game controls within its settings. Steam's own software can do this pretty well, but there should be support for that in every game up front.

Not only does it make it easier for people missing limbs or dexterity to play games, but it makes it easier for any person to tweak the controls for their play style.

I really hope we see more support for features like this because they can be so useful to everybody.

I think it would be hard to argue that having more accessibility options would be bad.

They're great, means I don't have to artificially adjust my play style to play with someone less experienced

I love them. I remember quake 3 had something similar too. It lets you challenge yourself a bit more, and in the case of Smash play equitably with players of different skill levels, so that everyone experiences a fun challenge.

Basically solves the problem of being "too good" to play with your friends

Being able to manipulate scores before the game starts is related to accessibility, and is always an optional feature. It helps balance the game or session for those who need it, or might spice up a game for those who enjoy it.

Aside from Mario multiplayer games (like Mario Party) and just sports games in general I don't know of any games that utilise this.

There are games that give you something OP it you fail a lot though. A lot of Nintendo games work like this when they notice you failing a lot and ask if you need a hand. There was another game I can't remember that makes you start with a power up if you die too many times, but I'll have to edit this post when I find it.

Edit: it is Super Mario 3D World / Land for WiiU / 3DS. If you fail a lot in succession the game throws you a golden leaf that makes you invincible when picked up. It's entirely optional otherwise.

Strategy games also tend to implicitly have it, in that you can team up the weaker player with a strong AI player.

Or sometimes there's also fun options, like a map where you can place the strong player into the fortified center and they have to defend against three weaker players at the same time. That can serve as a handicap, but the asymmetry also just means that it's less obvious and therefore less frustrating, who's better.

Generally, I'm in favor of having such handicap features, of course, but I feel like it's even better when the game's design is just naturally less brutally competitive.
For example, in Gang Beasts, yes, you're competing with each other, but the weirdo controls mean that it's never entirely your own fault when you lose, and of course, everything is just less serious in general.

Ultimately, such handicap features will break competition, too, because rather than the weirdo controls or your stupid AI buddy, you can then blame the handicap. I guess, it also helps to not take games too serious in the first place...

Lastly, I'd like to throw in the objectively best handicap: Having to play cooperatively with the weak player.
Just don't compete with each other, but rather tackle a challenge together.

When playing with friends/alone? Great! Customization is never a bad thing, and it enables groups with varied skill levels to still enjoy the game. Online multiplayer? Hate it hate it hate it.

One example: a lot of fps games are cross-platform these days, and I've never felt good about the things they do to balance mouse vs controller. I get why they attempt it, but it feels less like "balance" and more like they've created two different classes of players, controller being the close quarter players and mouse being the mid/long range flick shooters.

Another is any game that adjusts comeback mechanics during the course of a match, because I've never understood punishing someone for playing well

They are a great idea as long as they are optional and hopefully clear about what the settings do.

Don't really see it much because it's either single player game and thus it only matters to me. Or its a multiplayer game in which case it's just a new option for a group to agree on, on what is good.

Fwiw I've only really seen it in smash bros that I think of, and I think it's a great option. Let's me try out when a friend is toooo good and we give them less lives and stuff