For people willing to sign up for a personal use subscription (too difficult and not marketed to consumers, unfortunately), ArcGIS Online makes it easy to make really powerful maps, far beyond what Felt offers, for those interested. Newer things like blending modes are making the design aspect of map making a lot more accessible.
Video for those interested in what “more powerful than Felt” looks like: https://m.youtube.com/watch?v=0XI-J8XUNeU
And for developers, you can try out the samples to see what the tech can do and the code to do it. The flow renderer is one of the newer flashier features: https://developers.arcgis.com/javascript/latest/sample-code/... This shows how its done in code, but the public API underpins the Web Map Viewer, which exposes a UI for most of it.