Unique Non Clustered Column in Partitioned table
You cannot have an unique constraints backed by an aligned index (or plain unique non-clustered indexes) unless you add the partitioning column to the the unique expression. So if you have partitioned your table on column [datetime] then your unique constraint (or the unique index) must be ([datetime], [xyz]). Since more often than not this is not acceptable, the alternative are to:


remove the unique constraint from the data model (ie. accept that duplicates can occur)
keep the non-aligned index, with all switch-in/switch-out issues and performance problems