The slim ballast has a less than 1% failure rate. We sell more than 10,000 kits a month so if you see a warranty box full of ballast that failed, you will know why. With the full size ballast people don't usually buy bc of the size. Both are great kits, its just what type of money do you want to pay. I have the slim kit in another car that is not BMW, and it hasn't failed on me for 5 years, unfortunately I can't say the same on the full size ballast which failed on me once for my fog lights but now it is stable for the past 3 years.
Since it comes with warranty, you are set. Electronics are prone to fail no matter what, they can't last forever. If it fails, just warranty it out for a new one.