http://docwiki.embarcadero.com/RADStudio/Rio/en/Using_dbExpress_Datasets автор Many of the capabilities introduced by TDataSet are either unimplemented in unidirectional datasets, or cause them to raise exceptions. For example:

The only supported navigation methods are the First and Next methods. Most others raise exceptions. Some, such as the methods involved in bookmark support, simply do nothing.
There is no built-in support for editing because editing requires a buffer to hold the edits. The CanModify property is always False, so attempts to put the dataset into edit mode always fail. You can, however, use unidirectional datasets to update data using an SQL UPDATE command or provide conventional editing support by using a dbExpress-enabled client dataset or connecting the dataset to a client dataset.
There is no support for filters, because filters work with multiple records, which requires buffering. If you try to filter a unidirectional dataset, it raises an exception. Instead, all limits on what data appears must be imposed using the SQL command that defines the data for the dataset.
There is no support for lookup fields, which require buffering to hold multiple records containing lookup values. If you define a lookup field on a unidirectional dataset, it does not work properly.