So I figured I would chime in here since I had warranty work done(Hyundai seized my engine under an oil pressure test), a lot of it including engine replacement and turbo replacement, on my 2016 Veloster turbo. I was on the stage 0 tune (awesome btw) from Sxth Element Engineering the entire time this went on.
This tune was done at a meet where he removed my ECU, opened it up and used touch sensors to change the maps in my ECU. He explained at the time the only way Hyundai would know it was tuned is if they watched the maps in real time. This car was lemon lawed and was still giving me problems when I traded it in for my 2020 N line. I recently reached out to Sxth about tune options and specifically asked about the dealer being able to detect it. The head of their tuning department said they make no claims that it's undetectable because the dealership has a tool they can use to see the tune. I'm not sure if this in response to changing the way they do their tunes(it's done with a dongle).
So, while I have had great success in getting massive amounts of warranty work done on a tuned vehicle, I'm leaning more towards the Gan or scorcher units for my new car