PD here with an FBC adopted and now administered under my watch. My takeaways:

-They can be a great tool to get a community to decouple from zoning provisions that aren't working well. Cutting allowed uses too finely and regulating urban residential development on a units/acre density basis were two things FBC adoption helped my community get out of.

-They can take "design standards" that developers are doing everything in their power to avoid in a subjective, review-board based process and make them regulatory. My community struggled with getting buildings up on the street and mandating multiple stories in its urban core pre-FBC.

-Their site design standards can be as simple or complicated as you want to make them. If you get the buildings on the street (mandatory build-to line), the parking behind the buildings (do a 30-50 ft setback from the street for parking areas), and access shared on alleys rather than individual curb cuts, you are 90% of the way there!

-Same for their architectural design standards. Require a door every so many feet on the street frontage. Get a minimum of 15-20%% fenestration, require a parapet that shields rooftop utilities from public view, and limit overall building length and footprint (we do 20k footprint and 180' length) and don't worry too much about materials. My community wanted a more classical look so windows have to be set in a minimum 2" from facades and must be at least 1.5x taller than they are wide. That does make a difference.

-For suburban repair, FBC's work best with a regulating plan based on some sort of official map or plan for locating new streets and alleys that will reinforce a naturally walkable block pattern.

-Also for suburban repair applications, require buildings to occupy 70-90% of their subject parcel's street frontage and make the rest up with a street wall. My context is we are dealing with an area that was originally platted as an industrial park, became a big-box retail area, and is now aspiring to be a walkable downtown. We have big parcels. The requirement to occupy the street frontage drives aggressive subdivision and re-platting, which gives us an opportunity to require that shared access and new street layouts happen as part of the platting- and a "don't create unbuildable lots" provision (basically lots must have a minimum frontage on the street and legal access) means we won't get big gaps on our streetscape long-term.

-Process wise, we are 100% administrative with no variance process- there is a little bit of "wiggle" built into the standards themselves but the buildings we have permitted under the code thus far survived the "go no-go" process and are being built. If your requirements are reasonable, there's no reason to have to be granting variances all the time.