I'll just put it simply.
Most of the unreliability on the E60 surrounds the iDrive system and its buggyness. E60 are almost all associated with the i-drive and buggy software, which causes a myriad of problems from false warning lights to odd gearchange effects to faulty window operation etc..
The runflats are harsh and they suck, so get rid of them (if they are still on). Garbage tires, as are all runflats IMO. Electrical issues are main issues with window regulators, parking lights staying on too long, dead pixels on the screen, control system issues, etc etc. Too bad German cars don't have Japanese electrical components haha. Key is to keep on maintainence, make sure you do the waterpump when scheduled and do preventative maintainence. But it *should* be much more reliable than the unreliable E39. They are expensive on parts but they drive so well that it makes it worth it. Take it to an independent for repairs. A quick google search also confirms what I've said as well:
mine has been bulletproof, but its still under BMW warranty, only one issue that was covered, rain sensing wipers module failed ($800 part, 8 hours of labor apparently).