What is the problem of self assessment?

The main problem behind self assessment is the system built around it, reporting tax return each year can be frustration but nonetheless required, the body who deals with reported tax returns is HMRC. A tax return is the document required to report one individual's or organization's income however sometimes HMRC is not correct in dealing with matters and inquiries and thats where the real problem start. 

Client / tax payers scared and shocked by letters sent by HMRC about undeclared income.