There's no major downside to updating on multiple platforms, especially if you make it clear that there are other places to read it. I was actually encouraged to mirror my comic on other platforms by some people with very large Tapas followings (the kind that make decent amounts of money) after being initially hesitant for the reasons you listed as potential downsides.
If somebody only reads on Webtoon, they can read my comic, Errant, on Webtoon, but if they use both and prefer Tapas or prefer how I post Errant on Tapas (weekly updates of two pages rather than fortnightly updates of four), they can shuffle on over and read on Tapas. I don't really lose readers on either platform by providing something on the other, because die-hard readers are likely to use the main Tapas mirror, but casual ones who only read on webtoons probably wouldn't have found or read the comic otherwise.
Even with a lot of premium comics, the main source of income isn't Tapas or Webtoon but Patreon, Merch stores or print collection Kickstarters (or similar crowdfunding platform), so spreading out where the free readers enjoy the comic to cast a wider net isn't an issue if you also think of it as funneling people you've caught the attention of and who are willing to pay for more content or to support you making more towards those. The trick is to have lots of small income streams. It's annoying, but that's the life of a modern comics creator. 
I'd personally only post actual comic pages to Facebook or Instagram if I made short standalone gag strips. I'd put some kind of exclusive content or more of them paywalled on a Patreon and also make a bunch of merch. It could certainly work if your gag strips are memetic or relatable. I don't make gag strips, but I've seen this work great for some people.
