There are many happy owners with Ford 4.2 liter V6 engines. However, I have also heard about head gasket problems with this engine.
Consulting the Consumer reports history, I find that the Ford V6 as used in the Explorer has a better than average rating and only 2006 had cooling system problems. It could be that F150 owners overtax this engine , while other Ford product drivers treat is less severely.
In other words, I’m somewhat confused why your trusted mechanic calls this a dog. There are plenty of engines on the market that are real dogs and many posters can tell you about them.