There is no "best medium". There is only "what medium fits the particular story I want to tell?"
Webcomics are comics posted online, regardless of content. It's a description of how the reader finds the story, rather than the storytelling format itself. It's certainly not a "genre", just as "graphic novel" isn't a genre, but a description of format - the same way novel, short story, flash fiction and multi-book series are descriptions of length and shape of a story, rather than its thematic content. I mean, there are hilarious manga series out there, and graphic novels that make me laugh, so it's not like graphic novels have to be super-serious all the time.
I think there's a misconception that gets dragged along a lot when we talk about comics, because of the word's connection with "comedy" - it's assumed a comic has to be funny, because it's called a "comic". This is why graphic novels get called graphic novels instead of comics - it's in an effort to make people take the format more seriously, and treat it the way they would treat a prose-novel. To me, all comics - regardless of format - are comics. Newspaper funnies are comics. One Piece is a comic. Watchmen is a comic. They vary in length, form and content, but they're all stories told with sequental art and words.
I've read hilarious, gag-based webcomics - and I've read hysterically funny comics in print. I've read adventure-based webcomics - and adventure-based print-comics. Hwei's HERO: A story is a webcomic that fuses the visual aspects of comics with the focused power of narrative text to tell a very dramatic, deep and poetic story - and particular format only works as a webcomic as it uses hover-text for its narration, leaving the actual panels free of words.
A lot of very popular webcomics have very serious themes. Evan Dahm's work, for example, deals with a range of very heavy topics - like colonialist expansionism, oppression of native peoples, selfhood and the loss thereof, etc., etc. Dresden Codak is all about transhumanism, oppressive governments and absent parental figures.
... So, bottom line. I have to disagree with you on your definition of webcomics as lighter/gag-based, as I think it's too narrow.