I cannot comment on B12 as I have not used it. This week here on the forum is the first I have heard of it. I will probably try it though to investigate it.
I have used Seafoam and have recommend it many times when people have had minor problems with their CX/GL bikes. In many cases it saved them from having to rebuild their carbs and has also eliminated minor water problems in their gas tanks.
Is it a cure all ... no. Does it help? In many cases the answer would be yes. If the bike seems to run okay, but has hesitation on acceleration, it can help eliminate varnish that might be causing the problem. Sometimes it will even clear up problems that are a little more severe.
If the carbs are vanished/gunked up up badly only buying Larry's book and following the directions or sending the carbs off to Larry will help.
I use it from time to time in my fuel injected bike and car, just to keep the injectors clean and I have been able to tell a before and after difference in my car.
Take it for what it is worth ... it isn't a snake oil product, but it isn't a cure all either.