Franson is underrated if anything.
Out of all the dmen that have played 1000+ minutes over the past 3 season (230 dmen).. Franson has .83 pts/60 at 5 on 5 which ranks 55th on that list.
There is a pretty short list of Oiler dmen who are higher on that list... Klefbom ranked 34th with .89 pts/60.
Franson has played on a **** team with **** teammates... and has managed to stay above water offensively and defensively relative to his teammates over the past 3 seasons.
I don't particularly like Corsi stats at all... but he has put up strong relative corsi numbers as well over that timespan.
His GF/GA relative to teammates is also positive AND he's done that while getting a relatively strong dzone push as well... 4% more dzone than ozone starts.
His PP stats are decent as well... over the past 3 seasons of all dmen with 100+ PP minutes he's 53rd out of 127 with 3.77 pts/60. Dmen with similar PP stats: McDonagh 3.79, Parayko 3.79, Chara 3.78, Muzzin 3.77, Pietrangelo 3.77, Boychuk 3.76, Stralman 3.74.
That's pretty decent company he's keeping.
Add to that... decent size, RHD and he'd likely be cheap as chips at this point in the summer... he'd be a solid addition and I think there's a bias if someone actually thinks he wouldn't be good for the bottom pairing.
He's not an elite dman... but he's certainly a very good bottom pairing dman and bottom pairing dmen are still going to see 15+ minutes a night so that's a useful asset to have down there. Gryba/Davidson/Benning/Nurse all played 15+ minutes per game last season.