Hello Darren;
The reason most people limit the welding process to SMAW (or any particular process) is because they may have had a bad experience with the alternate process they prohibit. Many times the bad experience is a case of improper execution rather than a "bad" process and the bad experience is historical rather than recent.
FCAW electrode development has been nothing less than impressive over the last ten years. There is little reason not to consider the use of FCAW provided the proper electrode is selected.
SMAW can be a real problem if the wrong electrode is selected. For instance, if one was to use an E6010 electrode to weld a quench and tempered steel, delayed cold cracks would be a problem. However, low hydrogen electrodes that have been properly conditioned will provide the results required to ensure good service and freedom from delayed cracks.
Likewise, selecting the wrong FCAW electrode can provide unacceptable results. However, there are FCAW electrodes available that will provide excellent mechanical properties, good low temperature toughness, and low hydrogen characteristic. Their deposition rates can be several times that expected using SMAW.
It may be worthwhile to find out why your customer insists on using SMAW. Find out what your customer objections are relative to using either GMAW or FCAW. You may be able to present them with persuasive argument in favor of an alternate welding process if you have the proper information to substantiate your proposal to use an alternative welding process.
Best regards - Al