This is something I have mixed feelings about, and depending what the buffs entail. I feel I will most likely be against this kind of thing.
The buffs this gives out, should be very small, If at any time it ever feels like a requirement to join a guild to get some kind of buff, to be able to progress in the game. Then I feel whatever it is you added is too much. The buff should only be something that is kind of nice to have, but you can do with out it and be 100% fine. I also hope the buffs are not combat related.
Also I feel it is a major problem to have both buffs and the ability, to join as many guilds as you want. I feel this will create the meta, where there will be a “super guild” for lack of a better word. That absolutely everyone who cares about the buff, will be in, and set as their main guild just for the sake of having the top of the line buffs I have the odd feeling the portal seeker guild will most likely become this. Joining a guild should be about, being with a group of people who you respect, and have similar goals and Ideologys. it should NOT be because you just want some kind of bonus. so if you want to have buffs. I feel then you should only be allowed to join one guild.