Bluetooth's biggest problem for this application comes with the idea of pairing. The only one (sometimes two) device can be paired with a tablet or phone. If you want to listen to audio from the tablet and get engine monitoring inputs to the same tablet, Bluetooth isn't the answer.
USB's biggest drawback for this application is wire. There will need to be about 12 feet of USB cable from the front of the aircraft where the tablet would be to the back of the aircraft where the engine monitor would be (I have a Cozy that is a pusher). Putting a USB hub in the middle of the airplane would give me 2 six foot runs, but it still is inconvenient, and another point of failure.
WiFi, until recently, was a financial challenge. With Aurduino boards costing under $20 for what I want to do, and WiFi shields costing about $40 (sometimes they can be had for $30) it just didn't make sense. I am still trying to come up with a budget device that anyone can put in anything from a $10,000 homebuilt to a $500,000 production aircraft.
Recently a Chinese manufacturer has starting selling $3-5 wifi modules that will talk SPI and will run on the Arduino and other systems. The ESP8266 is available today for about $5. There are tutorials, libraries and sample applications for the Arduino. The module has an onboard processor that people are taking advantage of to make standalone projects, no Arduino required. It could be the thing that makes IoT a reality for many more consumer applications.
The module supports WiFi Direct (P2P) meaning I don't need a WiFi hub in the aircraft. The tablet or phone can talk directly with the module. If there is a hub in the aircraft, then that will work as well. It makes the whole wireless modules in the aircraft more of a reality.
I've ordered one of these modules, it may be a couple weeks before I get it. It will probably be a little while before I integrate it in the engine monitor as well. I'll keep you up to date on my progress.