As explained in the link:
1) The secondary research process can be completed rapidly – generally in 2 to 3 weeks.
2) Substantial useful secondary data can be collected in a matter of days by a skillful analyst.
3) When secondary data is available, the researcher need only locate the source of the data and extract the required information.
1) Secondary information pertinent to the research topic is either not available, or is only available in insufficient quantities.
2) Some secondary data [may not be accurate or reliable].
3) Even government publications and trade magazines statistics can be misleading. For example, many trade magazines survey their members to derive estimates of market size, market growth rate and purchasing patterns, then average out these results. Often these statistics are merely average opinions based on less than 10% of their members.