What is the issue?
Any chat addons which alter the color or format of the "[______] has come online" text break the greeting function of GuildGreet. For instance, ShestakUI's chat simply changes the color of the online/offline text.
The only easy way to get around the problem is to comment out the related code in the offending addon, which thankfully in the case of ShestakUI is simple, but needs to be done every time the addon is updated.
Thank you for your time, consideration, and updates =)