I noticed you tend to lean quite favorably towards Fellaini and a bit negatively towards Herrera. Which is fine, I think it's a bit of a Caf trend anyway. Some people "defend" Fellaini against some (IMO unwarranted at times) criticism and feel someone like Herrera is overrated, so they need to counter balance this by leaning marginally the other way. Not that I'm saying you're doing just that, mind.
I just had a quick look at whoscored's historical stats/ratings for the 2 players over those 2 seasons bolded above, and in fact the season before (Fellaini's first season with us and Herrera's season at Bilboa 2013/2014) and their ratings seem to balance out quite nicely, i.e. there hasn't been much to choose between the 2. I think they've both had periods where they were good, borderline excellent and relatively speaking, shit.
I also agree (disagree with whoscored - that last season Fellaini was better than Herrera. Ratings eh!). But in the end, I think it boils down to preference, style of play etc. Fellaini doesn't have a good reputation and IMO he's also
best used further up the pitch, almost as a 10, as opposed to a deep lying CM or even a DM.
It's actually a very interesting contrast between the 2 players because they have a completely different skillset.
On a final note, I've just started using whoscored and "stats" etc. So I'm not sure if they're a good measurement of a player's performance etc. But here are the 2 histories I viewed.
https://www.whoscored.com/Players/22738/History/Marouane-Fellaini
https://www.whoscored.com/Players/71174/History/Ander-Herrera