Excel files are an easy to use source in Qlik and there are no real limitations. The load-performance of xlsx will be lower as xls and those lower as from text-files but accessing data from a data-base isn't mandatory faster - it will depend on various parameters. Are the data-sets are rather larger with hundreds of thousands of records in the files and/or hundreds or thousands of files it may need some time - but regardless from the source this a good reason to apply an incremental approach.
Nevertheless there are a few specialities. Excel as source has no real NULL else NOTHING will be treated as EMPTY in default which has benefits as well as disadvantages and could be of course adjusted in various ways. Further complete empty records will be skipped and identically header get a continuously number added. Nearly everything what may occur is easy to handle.
Beside this you may have a more or less good data-quality but again this mustn't be more worse as by a data-base because they could also lack for mandatory fields and/or any validity checks as Excel could be manually manipulated.
