Serving up web content based on user agents alone seems exclusionary to me; mobile devices aren’t always constrained to low bandwidth.
To clarify, I’m not referring to responsiveness with regards to screen sizes across different devices, and it should be assumed the layout of key elements (navigation, calls to action, etc) is consistent across the low/high bandwidth versions.
The benefits include an optimised load time across all devices by default and relatively simple SEO (vs. breaking a site across multiple domains for mobile/desktop). But it’s not something you see a lot of in the wild, so I’d love to hear some thoughts.
I've mocked up an example of what I'm leaning towards, you can view it at https://www.nickdawes.dev/portfolio. This implementation allows the user to opt in to downloading additional content, which in this case is a full-screen background video and custom fonts. If they opt out, no additional content is served.