I barely remember seeing this show when I was a kid during the shows original run but I recently watched a couple of episodes on YouTube.
Apparently McLean Stevenson couldn't stomach being regulated as a supporting player even in a successful groundbreaking program such as M*A*S*H and chose to be a leading man in a rinky-dink generic family sitcom.
The McLean Stevenson Show wasn't a complete disaster, it was just completely unoriginal and filled with cliché's including wacky children and an abrasive Mother in Law. On the plus side, McLean seem to deliver his lines well, despite the weak scripts and the same could be said for the actors playing his wife and Mother in Law. However the performances of the adult "kids" and the grandchildren were pretty cringeworthy even by 70's sitcom standards.
Also this program had to be one of the cheapest sitcoms ever produced as the three camera show never seemed to venture out of the claustrophobic living room and kitchen sets, which had the stagecraft of a high school production. The walls literally wobbled whenever a character entered the room.
There is a YouTube clip of McLean appearing as a guest of the Tonight Show with Johnny Carson where he expressed displeasure with the direction of the show and its erratic scheduling. He said that if NBC picked it up for another season, there would be changes. Sadly the show was cancelled after a dozen episodes. All in all, it was probably the strongest of the McLean Stevenson flops but that's not saying much.