...believers should not be astonished...that sufferings strikes them. They should not consider it as if "something strange were happening." Such suffering is to be expected because its purpose is "to test you" (pros peirasmon). The NIV, unfortunately, leaves out the purpose altogether and hence fails to communicate why the readers should not be astonished.
Schriener's criticism of the NIV here is justified. It simply does not fully reflect the full intent of Peter's words in the Greek:
Dear friends, do not be surprised at the painful trial you are suffering, as though something strange were happening to you. (1 Pet 4:12 NIV)
Does the TNIV improve upon its predecessor? Absolutely.
Dear friends, do not be surprised at the fiery ordeal that has come on you to test you, as though something strange were happening to you. (1 Pet 4:12 TNIV)
Not only does the TNIV correctly communicate the purpose of the suffering "to test you" (πρὸς πειρασμὸν/pros peirasmon), it also renders πύρωσις/purosis correctly as "fiery ordeal" rather than the NIV's weaker "painful trial." Thus, Peter's allusion to the OT idea of a "trial by fire" (Prov 27:21; Psalm 66:10; Zech 13:9; Mal 3:1-4) which was all but absent from the NIV rendering is clearly presented in the TNIV.
The TNIV improves upon the NIV not only in regard to the changes in the English language over the last generation, but more importantly in terms of accurately reflecting the original biblical texts. From this point forward, publishers who are considering the NIV as a basis for a commentary or reference work would do well to choose the more accurate TNIV instead.