So I was working on a character for a Shadowrun 5e game and specifically did not buy a helmet because wearing a big, bulky combat helmet is extremely noticeable and I figured there might even be social stigma against wearing one around all the time. However one of the other fellow players heavily suggested it. I wanted to know what the pros of wearing a helmet in combat are (beyond the simple armor number raise) and if there were any mechanical cons to wearing helmets. Thanks.
Helmets can be modified to include vision and audio enhancement. The full body armor's helmet for instance have a capacity of 6 (which is appreciable).
4th edition had a rule for targeting a spot not covered in armor and I think it's still valid and interesting in 5th edition.
Target an area not protected by armor. The attacking character receives a negative dice pool modifier equal to the target’s armor (better armor is more difficult to bypass). If the attack hits, the target’s armor is ignored for the damage resistance test; the target rolls only Body.
This way having a full armor with a full helmet would makes it (almost) impossible (-18) to do. Which makes sense to me. At this point I would probably just call a shot to the head and hope to pierce the metal.
This is only if your GM is using this rule from 4th edition (which I expect to see back in the Arsenal book eventually).
Like Scrollmaster mentionned, there's also the fact that helmets masks your face but it's probably not suitable with some dress codes. Don't try to get in Dante's Inferno with anything else than a nice suit or dress.