As I'm really enthusiastic about UIkit (people who know me can confirm that) because it's so more aesthetic compared to Bootstrap and moreover loads less resources and doesn't need for example jQuery to work properly.
After adding the UIkit framework besides some fonts styles that were overruled – nothing noticeable about the look & feel happend - which is in this phase very good! As UIkit uses the "uk-" prefix before all classnames it can run perfectly next to another framework like Bootstrap, they don't bite each other much.
What's so cool about UIkit (version 3 at the moment) – is that it's not that bloated with tons of classes, but is really a well thought frontend framework with a logical and consistent amount of modern classes and attributes which make sense.
Compared to for example Google's Material Design (and also Bootstrap) it's not pushing its own look & feel that much, but the elements and components are pretty 'vanilla' – they look aesthetically very clean and are easy to style with your own design.
At the moment I'm at approximately 80% of migrating to UIkit and as my website is on MODX, this goes very modular – with all code residing in the CMS. At the same time I try to rehaul some components which I use a lot (image-boxes, lighboxes, overviews of items and templates) to my MPS approach so I can use them throughout the website in a modular way.
This will result in less code, which means more efficiency and consistency throughout the partials and pages – which are almost 300 indexed and still growing!
Also this modular approach results in that I can reorganise my old CSS code very easy to Less CSS format and mixins very easy.
All together a very interesting process which I'll keep you posted about!