An inline six, thermally, is inferior to a V6. Fuel economy typically is poorer. Don't get me wrong though.... it's my preferred engine config... perfect primary and secondary balance. No balance shafts necessary. Wonderful sound too. Only V6 i really like is the long-ago Alfa Romeo SOHC V6... notwithstanding its wet liner and other issues hehe. Amazingly great sound for a v6.
That's correct, the rear of the engine, closest to the engine bay (cylinder 5/6) gets very hot. When i was changing spark plugs on my BMW, the plugs closest to the back looked really bad and worn. The early model 335i BMW's back in 2007 would suffer from limp mode during track use because those cylinders in the rear would get way too hot. Turbocharging only makes this issue worse.
Inline 6's most certainly are not
less efficient than a V6 though. If Inline 6's were as common as a V6 or even somewhat more common than they are now, we would certainly have a better idea. Their excellent natural balance gives them ability to make strong linear torque from low RPM's which in turn allows you to calibrate the transmission more precisely. When engineered and tuned correctly, they can be amazingly efficient and capable.
The main benefit that V6 engines have really is packaging. Since they take up less space length-wise VS. Inline 6 engines, you can fit it into virtually any car. Nissan for example has been using the same family of VQ engines since 1995 with practically zero updates and shove this V6 into everything from Altima's to Murano's to Infiniti G37's. An Inline 6 only makes sense for brands like BMW who despise FWD applications. Also, since V6 engines have two smaller crankshafts, this allows the engine to rev up quicker from low RPM's. Inline 6 engines have a very heavy one piece crankshaft which hurts free-revving in the low RPM's and also lowers the redline. I don't how how BMW does it... incredibly smooth, rev happy motor, 221 FT/LB of torque from 2750RPM in a 3L engine, 7000RPM redline, and 32MPG on the highway in a 3900LB luxury boat. It's really quite a feat.