Hi,
I believe the constratins vary from target to target (software targets and hardware (ASIC) targets). This seems a question for the Intel forum. Have you tried to ask it there? See a link here for the forum: PHV error on if condition - #2 by andyfingerhut
In general terms, software switches are more flexible than hardware targets, but with great flexibility comes, not as great, line rate processing capability 
Cheers,