I agree that what's most attractive about him is the system fit. The Rams took a
very poor approach to trying to fill the QB2 spot last offseason. And between Stafford remaining healthy and finding Wentz later in the year thankfully it didn't come back to bite them.
Now every backup QB has flaws & issues and inconsistencies, otherwise they wouldn't be backups. But how many other backup QBs out there would we rather have? A handful? Maybe Browning, Minshew and Flacco? I like JG with the Rams better than, say, Dalton, Darnold and Brissett (the latter two will start at least the first several games for their teams).
Both for his career and over just the past three seasons he's won over 60% of his starts. And his QB rating- again both career and just over the past 3 years- is over 96. I do agree he's been unimpressive as a playoff starter. Hopefully he would not need to start a playoff game for the Rams...
Look at what McVay was able to do with Mayfield and then Wentz. Maybe he can work similarly well with JG.