You are correct. _.uniqueId() will absolutely wreck your list rendering performance if it is called on every render. 99% of the time, you can find a better unique identifier, but if you absolutely need something like _.uniqueId(), it should never be called in the render loop. It should be attached to the data being iterated over, in advance.
But... the other problem with _.uniqueId is that in an isomorphic setup your server will just keep counting up and up while your client will start from 0 on every page load. So the first time you reinflate React on the client side the checksums won't match and will cause a full-page re-render.
I only do that on the client, and yes, as a matter of facts, things are rerendered each time.
But we are talking of a couple uls with <10 elements, frankly the eye doesn't catch anything at all...
Honestly, the real damage of this bad habit of mine show off when I've to explain to my co-workers why it could be bad to do this for every situation.
But... the other problem with _.uniqueId is that in an isomorphic setup your server will just keep counting up and up while your client will start from 0 on every page load. So the first time you reinflate React on the client side the checksums won't match and will cause a full-page re-render.