Will UpdraftPlus leave large temporary files behind like other backup plugins?

The short answer is that UpdraftPlus cleans up (removes) all temporary files that are more than 24 hours old, so you should not need to do so manually. If you spot any temporary files that are taking up a lot of space, then they will be less than 24 hours old, and disappear by themselves later on.  More information on how UpdraftPlus temporarily uses disk space is available in this FAQ.