The short version: most holds are documentation queries, most resolve, and the paperwork description is the variable that predicts them.
Most holds are documentation queries: an incomplete description, a missing value, an unclear commodity code. They generate a request for information and resolve when it is supplied.
Suppliers differ in how they complete documentation and members have reported markedly different lane behaviour as a result. Asking how a consignment will be described is a reasonable pre-order question.
Personal and commercial importation are distinguished in most customs regimes by criteria including value, quantity and consignee.
A false declaration is a separate offence in every regime and is never the right response to a query.
A seizure notice has a deadline. Read it properly and take it seriously.