Well, technically when a dev makes an app without designer, he is designer himself.
But I'm talking about when you are skilled UI designer, making designs in Sketch/Figma, then coding them.
Just curious, since this is what I'm trying to do when we cannot afford a designer in our small company and it doesn't seem to be very hard task. I mean, UI is not an art or drawing illustrations...
Oh, you would be surprised. Anyone can make an UI, but it takes talent to make a good one. Plenty of modern websites are in desperate need of someone who has read "Don't make me think" [1].
I have combined both roles in small projects, and following some conventions about UI design has gotten me far. Modern design tools also nudge you in the right direction, guaranteeing that the result is fine as long as you don't fight against the tool. It may not be excellent UI design, but it's appropriate UI dsign and that's more than half the battle.
Might be a bit biased because I've been able to excel in the space of designer who can also code their own prototypes and can talk to devs on their own terms. But personally I believe there is more potential in leaning more design side, few people have this skill set and if you find a company/build a company that appreciates it you can pretty much control the narrative because whatever direction you want to go you can just spin up enough of a prototype to sell the concept to the stakeholders yet also have the luxury of being too busy with design work to actually do deep end engineering.
Just being extremely honest with my own strategies here.
if I design something it is good maybe 1/2 of the time, if it is not good I have to retry.