I have both Peake Tools, which are well made, compact and reliable but the two combined will only access the DME (Engine Control) and SRS (Airbag) modules - they will not access other critical modules like the ZKE (Central Body), ABS etc.,
I also have Carsoft, which accesses more modules (but not all) and can be set up on a laptop with free software and a $59 interface and cables.
Apart from Carsoft and the Peake Tools, I have INPA and DIS (software can also be obtained free), which give dealer-level diagnostic ability and can be used with a top of the line interface on a laptop for around $150 (the 2 Peake tools cost me nearly $300).
INPA and DIS can also access every module in your car.
The only reason I can see for going with the Peake Tools is that they are self-contained (i.e. do not need to rely on software running on a PC or laptop). As far as functionality goes, they are fine but limited - you can get far better value for your diagnostic buck.
If the DME is dead, it doesn't matter whether there is power to the diagnostic port or not, no fault codes will be generated.
Apart from the above diagnostic tools, I also own an Autel Maxidiag and an EQUUS Scan tool, but for BMWs, I would choose the laptop based systems any time.