Webtoons have a different economic model than Manhwas. They're usually published weekly and the graphic quality is usually lower for lack of time. This being said, some are visually interesting though the fact that it's destined to be read on a screen is an important limiting factor. We see the world in "widescreen" not "vertical screen" and action scenes in webtoons are challenging. Also as mentionned by redfox, the composition is different. In webtoons, you compose one image and than the next, in manga, manhwas, comics (US or Europe) composition also take into account the medium. It's not what's in a single panel, it's also how all the panels on a same page or double page are arranged together. Mangas in particular have taken that to a whole new level, I've yet to see a manhwas that manages to pull this off, though I'm sure they exists somewhere.
Also, the difference is that unlike manhwas, only the extremely popular ones get published which means that as it's not costly: there's a whole lot more of webtoons out there... and some of the art can be pretty awful all the while being popular series. Manga authors published weekly also have assistants but, I'd argue that the graphic quality is usually higher. Comes from the fact that they're working in black and white. If you work in colors (though some webtoons are in black and white), mistakes, errors and texture discrepancies are more visible. It also take more time to work in details even when you are experienced. Most webtoons are in flat colors i.e no variations on colors. I'm not a US comics reader but I've read some and it varies, same with european comics.
So basically, I'd say it's just as varied as any other genre, it's just that there are more bad available than good given how cheap it is to upload it online. I've stumbled upon some extremely well drawn sport webtoons on Naver in colors with variations in values, tones and textures and apart from some panels which action were limited because of the verticality of the screen ( I remember reading a bike scene vertically once, the author had drawn two bikes competing again each other and he didn't bother, he drew it than turned it vertically), it had absolutely nothing to envy to its paper counterpart.

In terms of quality of plots though, I don't think you can make a judgement based on the genre. Whether we're talking about US comics, mangas, manhwas, webtoons, or european comics... It's just as varied as what you would find in the film industry or in literature.
It's just a matter of you liking the medium or not.

As far as I'm concerned, I've got to admit I'm not too fond of webtoons. Though I've started to read some a couple of months ago because I found it was an easy way for me acquire vocabulary in Korean. I've yet to warm up to the genre though... It's just not the same as reading a book though.