I want the users who download and install my extension from the marketplace to go through a registration/login flow. Since the normal system.xml file supports only static pages and doing the entire thing using FE models is too complicated, I'm wondering if I can instead show everything in an iframe?

Showing everything in an iframe gives me lot of advantages like -

Flexibility over design. Magento buttons/elements are ugly.
Being able to use the same iframe my other eCommerce offerings
Much much less engineering time since our team is much more comfortable with JavaScript/CSS than PHP/XML etc

So my question is how to implement the iframe and are there any disadvantages that I'm missing?

I see its possible for Magento 1

magento2 configuration system-config iframe

default