Macro editing consequences

Hi to everyone.

I wish to know some pros and cons of having lines of code written in a macro.

Is this something obvious to do? Should one avoid it and if yes why?
Some where I ve read that one should keep macros as short as possible, if yes why?
Should one make time delays when editing macro files, or are delays likely to cause problems?

I am more concerned on editing the M3 macro file. Is it true that the M3,4 and 5 have higher priorities in mach 3 and work somewhat different as in if seen at a low level?

Can someone point me some general guidelines when macros are edited.

Thank you all in advance.