I like both Shad and GIVI, for me it mostly comes down to design and "fit" of the look on a particular bike / scooter.
SHAD does have brake lights but theirs are on the rack not the case, so they sit just under the case. That said I would not look at the Givi or Shad brake lights as a buy or not buy issue. I thnk both are limited... The Givis have lot's of connection issues, ie they don't always make contact so no lights some times and both Givi and Shad are only break lights.
Personally I would buy the case you like and add an "Admore Lighting" kit to it. Their kits give you tail, break and turn signals.
I love the light on my SHAD, and those are the exact reasons why, since it stays on the bike at all times and there's no
connection issues. It's also brighter then the stock light so that's great too.
I like the build quality of the top of the line SH50 shad better then both the kappa and givi top of the line stuff, but it's a toss-up
in lower end models. Like you said, people tend to buy what fits the bike best visually, and I don't think the givi 55l maxia works
well.. but I love how the shad sh48 and sh50 fit (sh48 even better).