Application forms and claims nearly always want one attachment, not five. Joining the documents first is usually the difference between a submission that goes through and one that bounces.
Files almost never arrive in the order you want them. A cover letter that sorts alphabetically after a bank statement will end up in the middle of your document, and nobody checks until the form has already been submitted.
Merging does not re-compress anything, so the finished file is roughly the size of the parts added together. If the combined document is now over an upload limit, compress it after merging rather than before, because compressing each piece separately gives you less control over the final number.
Pages keep whatever rotation they had. If one of the source documents was scanned sideways, it will still be sideways in the merged file, so fix the orientation first if you want a tidy result.
There is no limit set by us. Very large jobs are limited only by your device's memory.
Turn the photos into a PDF first, then merge that with your document.
No. The pages are copied across as they are.