I see the mind trick as more of a gradient difficulty depending on several factors, which include but are not limited to the weak-mindedness of the subject(s), the number of subjects, and how much you want the subjects to deviate from their intentions. On a scale of Not Really Tricky to Impossibly Tricky:

For Obi-wan vs the stormtroopers, it probably would have been easy to sway their minds and also to have them simply overlook a mildly suspicious pair of droids. However since it’s a group of Stormtroopers, he would have had to mind trick all of them to some extent for it to work. Moderately Tricky

Luke vs Bib Fortuna was probably on the easier side since it was just the one lackey, but not a walk in the park since he had to overrule a direct order. Mildly Tricky to Moderately Tricky depending on the Bib’s SAT score.

Qui-Gon vs Watto is right out due to biology. Impossibly Tricky

Rey vs Daniel Craig would seem simple because it’s just one trooper with a presumably weak mind, but it also goes directly against his orders. Mildly Tricky