Hi Julius,

This is an interesting question though
Alright, let me share my opinion...

Zurb Foundation is the second most popular front-end framework and UIkit is the third most popular. The most popular one is.... yes, Bootstrap.
In my personal opinion, UIkit is the best. It is very easy and intuitive, it gives you more features than the others and at the same time it is lighter in size. The UIkit (and its components) that comes with our templates is about 200KB - this is the size of an image and usually a website has 10s of images!

Now, using UIkit features is really, really easy. You just need to copy the code from their documentation and place it wherever you want on your website. Here, have a look at this blog post and make sure that you watch the UIkit demo video. It shows most of the things that you need to know.
Regarding the UIkit course on OSTraining - I just had a look at the different lessons and I do not think that it explains everything. Yes, of course it will be helpful if you take the course, but in my opinion you will learn UIkit faster and better if you read the above blog post, watch the demo video and then just practice the UIkit markup (code snippets) for couple of days.

Regarding Foundation - you can keep learning it, even though you will not be able to use this knowledge with our templates.
In my opinion, the front-end frameworks worth learning are only UIkit and Bootstrap. This is just my opinion, I might very well be wrong
Anyway, if you have some knowledge in Foundation, it will be very easy for you to start using UIkit, because all front-end frameworks work in pretty much the same way. Only the names of the classes and the data attributes are different