I'd call the Fenway deal largely a failure and here's why:
John Henry never uses his media outlets to promote his NASCAR venture. Fenway owns 80% of NESN but you never really see NASCAR covered at all on there. Outside of northern New England, you never see NASCAR promoted in the region period. This could've been a good chance to expose NASCAR to New England but Fenway hasn't capitalized. Fenway Park SHOULD have ads for NASCAR, RFR and the drivers (in some capacity). Red Sox fans are loyal to Fenway so you could've converted a few of them into RFR fans.
If Roush were smart, he would've tried to sell John Henry on paying big bucks to sign and sponsor the only native New Englander in the field (Joey Logano) so fans from the region have a driver on what is essentially their home team.
In fact, I have the NESN app on my phone and there are tabs for the Bruins, Red Sox and Patriots but none for NASCAR. RFR is essentially a sister org to NESN but gets no coverage from them.
I assume they've been a financial help though and that's important to them now.