for those stuck on what to do when unity refuses to open a project with ‘waiting for unity .dll to finish compiling’ this isnt actually a problem with unity, its related to the unity version control (plastic SCM) to fix it, all you need to do is open plastic SCM and follow the prompts to login to unity, and bam.
i ran into this again today despite updating a bit back so for anyone banging their heads against this for a week like i did, here you go