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I used JB4 for about 2 years on my m240i. Didn’t have any issues with it except during hot summers in Florida where the engine would get heat soaked and then boost would hit strong and not consistent (some days it’s feel powerful like normal, other days it felt like it was sluggish. Because it only monitors 2 sensors on the b58, it makes it difficult to accommodate boost during heat soak. However, it’s a great option because it can be removed before dealer visits and makes it harder for them to notice performance was modified (an engineer would need to deep dive into the files to notice overboost from the standard factory setting, something the bmw technician at the dealer wouldn’t readily notice from a scan).
BM3 is a complete flash of the ECU, therefore it can accommodate different parameters and adjustments for heat soak and other climates, fuels, etc. more adjustability here and because of that it is a safer option overall and can usually produce better performance gains. However, it is easier for the BMW technician to identify performance modification because part of their scans identifies firmware version, date it was flashed, how many flash cycles the ECU has undergone, and ignition counts since last flash. All variables that flashing back to stock and relocating the ECU will not change. If you go in with a blown head gasket or rod knock, the first thing they will do is run this scan and if any of those numbers seems off, they will instantly void your warranty. For example, the scan shows it was flashed to the current version 2 days ago, with 10 ignition cycles since flash, but their records show it was last flashed (either by dealer or factory) months prior. They will identify this as modified and void the warranty right there.
There’s inherent risk to performance modifications, whether it’s a piggy back or a flash. With all of this in mind, for me personally, I’m sticking with BM3 mainly because of my location and my performance goals.
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