I also think that the guilds are too powerful.
In fact there are two problems.
1/ 12 trade is not a big amount, yes. But for a size 1 city, with the only requirement to build a very cheap unit, it is.
2/ AI *never* use guilds (at least, it seems to me). So it is presently totally unbalanced and guilds give an incredible advantage to human players.
To my opinion, the best guild in terms of balance is "farmer". You have a small food bonus (never saw more than 2 or 3 even with a huge empire), but you have access to a nice trade enhancing building that you must *build* in your city.
I wonder if all the guilds should not be done this way. Divide by 2 or 3 the initial bonus, but if you want, you can build a powerful building in your town to increase trade, culture, production, etc, depending on the guild.
This would avoid that small cities have an insane amount of culture, trade or production from scratch. And to have an extra advantage, the city should work to construct the adapted building. More, presently the guilds provide a uniform benefit. This way, the larger the city, the larger the advantage they can get from a guild (with seems to really make sense IMHO).