The Sky Marshall program in the US is working quite smoothly after a few start-up problems. It is not perfect but it is working.
The Captain always knows if a Sky Marshall is on board. Both parties know their limits and duties. There is no real additional conflict with Captain's authority since historically there have been similar cases of federal authority over-riding the Captain. The FAMs have been taught about Captain's authority and they are usually pleasingly polite. The FAM/Crew working relationship seems quite good.
Some points from earlier posts. Some posters imagine there is a reserve of people somewhere experienced in FAM type scenarios. Not true. The aircraft scenario is sufficiently different from ANYTHING that previously existed, military or civilian, that a whole new training course is required, even for experienced law enforcement personnel. There is no reason a person from any previous discipline should not allowed to be a FAM, presuming that person passes the required training course. Myself, I think self-discipline, cunning and timing are probably at least as important as firearms ability and strength.
There is no credible replacement for a firearm (reasons already explained on previous posts). Within the range of available firearms and ammunition there are better or worse choices for the job. The FAM program has selected a reasonable compromise in hardware.