There should't. But if your machine is broke in some way i would't tell them you used custom Firmware.
And before contacting support. I would place the original SD-Card back and test if the problems is coming from the Image (or SD-Card).
You should maybe keep your Stern-SD card and put the image on a different SD card.
Using a diffrent SD-Card should not be a violation of the warranty. They ship out the image for you to put on SD-Cards. The original SD-Cards from stern are cheap and tend to break in some time.
What the guys are doing here is "only" replacing existing sound and video files with custom ones.
This does not alter much more. The worst thing that could be happening would be things that could also get of broken SD-Card from stern in my opinion.
i would't put an image in my machine from an untrustworthy source.
The guys here seem to be trustworthy, a lot of guys test the Images. The images add a lot of the experience. It's like playing a different machine.
It's a lot of work they are doing and i am glad they are sharing it.