Hi David,
The manufacturers will claim that Wi-Fi is not a problem. The problem is that in the real world, especially at modern festivals, there are all kinds of data flying around at 2.4 gigahertz including Wireless DMX for lighting, Bluetooth, and often times dozens and dozens of what they are now calling micro access points around the venue that allow the festival to serve content to all the mobile phone users in the audience. Each of these services are extremely low-power and often don't cause any interference to more powerful microphone or instrument systems when they are the only source of noise. However when you have all of that stuff simultaneously clamoring for the same slice of spectrum, the noise floor rises significantly.
What's worse is most of these devices use self coordinating, or frequency hopping, or other wideband transmission schemes that are completely unpredictable when attempting to coordinate your instrument Wireless Systems. This leaves you vulnerable to interference.
The bottom line is that if you are a professional playing anything beyond the level of the same bar gig or church every weekend, in the same venue or familiar venues, 2.4 GHz is not appropriate.
Sent from my mobile phone. Please excuse the inevitable spelling and grammatical errors.