As a GM, basically any artificer / inventor. They only fit into very specific settings, so they’re very out of place in most games. If the system has light rules for inventions, the player thinks they can create anything, and I have to constantly fight them to stop trying to one-up the other characters. If the system has robust invention rules, these characters don’t generally get to invent anything since so much downtime and resources are required.
From the player's perspective this is a rough one as well. There's nothing more disappointing than to roll up a crafty character only to discover that the campaign has break-neck pacing to prevent rest spam, but also incidentally preventing any downtime for crafts.
This was a problem for Mad Scientists in Deadlands. Some builds took months or years to create, and when time is of the essence, no new toys for you, scientist!
Very much this. It's basically the "hacker movie" problem in tabletop form. Actual making involves a ton of time and most of it is boring (even if the results are amazing). It's very difficult to translate this into the pace of a story while still making it interesting. To do so you often have to engage in flights of complete fancy, like the competitive code writing scenes in hacker movies.
It's especially bad in D&D5e, where the artificer can create any common magical item, but it has to be selected at level up and can't be changed, and since the game is so focused on High Fantasy, all of the common magic items are completely worthless, since the interesting stuff happens at higher rarety.
In the end the system makes the Artificer a reskinned magic user where everything is worse than a plain sourcerer.