Salesforce Bucket Field Limitations in Reporting

A Bucket field is a report field that groups values into buckets. You can then display this new grouped value by name. For example, start or view a report on Accounts.  You can then create a Bucket column named Emp Size based on the Number of Employees field.  The new bucket field can group records into Small, Medium, or Large that you define for accounts. 

Information on how to create a Bucket column is available in the Salesforce Help Categorize Data with Bucket Columns. 

The following are the limits for Bucket fields and the errors that can occur when one of the limits is reached. 

Limits

• Each report can include up to 5 Bucket fields.
• Each Bucket Field can contain up to 20 named groupings. 
• Each grouping can contain up to 20 selected values. (Doesn't include the use of "Other" as permitted within the Bucket Field setup)
• Total values allowed per Bucket Field - There is an overall query limit that is generally hit when bucket fields contain approximately 42–45 values. Surpassing this limit results in an error message: "The query for these buckets is too complex. Remove one or more buckets and retry."
• Editions: Available only in Developer, Enterprise, Performance, and Unlimited Editions 

Limitations

• "Query is too complex" error - Whenever you've exceeded the overall Bucket Field limit (usually around 42-45 values for the whole bucket), you receive an error stating, "The query for these buckets is too complex. Remove one or more buckets and retry."
• Reusing Bucket Fields in multiple reports - Bucket Fields can only be used in the report where they were generated. To use a Bucket Field in another report, you need to re-create it for each additional report or create a separate formula field for the object dependent on the bucket.
• Custom Summary Formulas - Bucket Fields cannot be created against Custom Summary Formulas. 
• Reporting Snapshots - Bucket Fields are available for Reporting Snapshots only if the source report is tabular.
• Historical trend reporting - Bucket Fields cannot be created in Historical Trending reports.
• Record Type - The standard Record Type field cannot be bucketed. 
• Bucket size - Buckets with values that exceed more than 1,000 characters in aggregate do not appear in dashboard components when used in the "Group By" field.
• Null Values - In a Numeric Bucket, you can move all empty or null values to the bucket that contains the value zero by   selecting Treat empty values in the report as zeroes. If deselected, unbucketed values appear as a dash (-).

More limitations are available in Add a Bucket Column.