He took over Burton in the playoffs and went up as champions in League 2. Left them top of league 1 I think?
Qpr had Chris Ramsey and then Neil Warnock after Harry Redknapp. Was different from the premiership team having lost effectively their entire first team squad upon relegation (Since JFH, they are only 1 point above where JFH left them).
Besides that... you are right that he has never been involved in a real relegation scrap. But he currently does have his hands tied with such poor attacking options available to him. The ones who actually look like scoring are currently injured.
He didn't have to do much with Burton, just carried on the motion set in place by Gary Rowett.
At QPR, Chris Ramsay suffered from the Harry Redknapp aftershock whilst Neil Warnock was only in post for less than a month.
JFH was sacked after 11 months, not being able to do anything about the situation despite having two transfer windows and funds available.
He will have at least one transfer window to put us right, I hope he does but as you have acknowledged his CV doesn't show a successful experience of this aspect of management previously. I sincerely hope that he has learnt from his previous experiences for the good and benefit of NTFC.