We already know that "What is your worst miscast?" is a common apprentice game.
The point of the Journeyman trials is to demonstrate that you have the capability and maturity and desire to handle unsupervised magic use, and that you can be trusted as a sanctioned magic user, in the distinctly not-fully-safe Empire.
If the trials had no dangers, they wouldn't be an adequate test to prepare you for the rest of your life as a trusted Magister, and the obligations thereof.
And if you do not feel ready for that? You can avoid all that, stay as a Journeyman up to and including the rest of your life - no one is systematically forcing wizards to deadly tests, beyond what deadly tests already come inherent with being a magic user in Warhammer Fantasy.
And for apprentices that don't even want that? Wasn't there some options for being taught enough to not turn yourself into an active danger to others, and having some suppressants or something and then being let go as basically your average citizen?