I switched from material design (react-toolbox) to semantic ui around a year or two ago for similar reasons.

Material design looks great by default but it infects the rest of your app really quickly since any custom components you build have to match it in visual style or it'll look really out of place. This was pretty hard for a small project without a designer.

Semantic basically looks like bootstrap without being bootstrap and is much easier to work with.

The main dislike I had of using Material Design was trying to make it not look like just another Material App.

With bootstrap the basic buttons are fairly simple, so distinctive theming was easy, but with all the shadow and shimmer effects of the Material Design components its a lot of work if you want to change their structure to give it some of your own app's personality.

I tried both at the beginning of 2016, and neither of them seemed very usable at that stage. Both look much better now, and I can't wait to give them a shot.

Also noting that the semantic UI docs still give a horrible mobile experience (iOS 11).

I've been using SUIR in production for about a year now. It's been working out pretty well.

Semantic UI is anything but lightweight.

Yeah, it feels more heavy to me as well.

I meant lightweight visually

Semantic UI React is amazing. I've used it to build a number of applications. My favorite thing is the interoperability between its various components.