The search space even for the less secure mini private key is still massive (58^22, I guess?). Any attempts at cracking would be an academic curiosity rather than a way to make (ie: steal) funds.
There shouldn't technically be any difference between strength of a 22-word mini private key and a regular bitcoin private key since both of them provide the same level of security (128-bits).
58^22=6.2e+38 and 2^128=3.4e+38
Keep in mind that the mini keys act as the entropy used to derive the key from.