Source is really more a mini-framework than a full-blown one. It’s good, fast, great support and gives you a great base from which to build sites using your favourite 3rd party stack.
I have Platform, and used it once, but didn’t expand on its use. Not a reflection on it, per se, it just didn’t hit the mark for me personally.
Uikit (3) is my weapon of choice in the RW world. In my opinion, it’s the best out there, but only in so far as it works the way I like to work. It has a very complete range of stacks that when used together, can accomplish pretty much everything you need to build complete sites. Add in the huge range of classes that are baked in, and it results in a framework that is super fast, incredibly adaptable and extremely capable. Support is also excellent.
UIkit does have a learning curve, though. I don’t use F6, but from what I hear, the curve isn’t as steep, but there is no doubt you do need to put some effort in to get the best out of it.
To give you an example of what I mean, let’s say you want to make a full size (100% browser width and height) “hero” header. Most frameworks have a single stack for doing this. UIkit takes a different approach. There is a multitude of ways to do it using a different mixture of the included stacks, depending on precisely the end result you want. The upshot of this is UIkit is less “drag and drop” of a single stack to do a single task, the upside though of this approach is that you have a lot more control over the end result than most other frameworks.
I use UIkit to make some pretty standard sites, my portfolio is here: https://www.caffeineinjection.com/portfolio/, but I’ve seen some stunning sites built with it. So it’s not just about “regular” sites.
If you want a reasonably simple framework to get going with, Foundry is a great place to start. Whilst @Lucas the dev of UIkit has handed the user as much control as possible. @Elixir the dev of Foundry has taken the opposite approach, restricting what can be done, in an effort to allow the users to get great looking sites built quickly with the minimum of head-scratching moments.