You've got another problem to resolve first:
I'm guessing you have this relationship:
User>---BusinessUnit-----<Projects----<Tasks (---< means one to many ---- means one to one)
If so, I don't think User::username = Get ( AccountName ) will work to control access to Projects. If you have more than one user record linked to BusinessUnit, the link from Projects to businessUnit to User will match to the first User for that BusinessUnit and this may not be the user who logged in and this will then deny access to the user even if they are listed as part of this BusinessUnit.
If John, Jim, and Janet are listed as members of the "Sales" business unit, this expression will not allow Jim and Janet access to the Sales projects, only John will have access, as he is the first user listed for this business unit.
This method will only work if there is only one User record assigned to any given BusniessUnit record.