The show seems to have been put through a focus group and has been watered down as a result. All the subtleties have gone. Where it used to show the parallels between the characters' every day lives and the serials killers they study, they now just make it super-obvious.
Clearly people were asked who was your favourite character? And so this season Bill Tench is the main character. That's not really a problem but without Holden to contrast and balance him out he doesn't seem as interesting.
The show was hugely interesting because it was exploring the extremes of humanity and we could see its effects on the 'normal' characters. Now it's more about the characters (who just happen to interview criminals). In shifting focus has become just like very other character drama show. It's lost its point of distinction and become less interesting.
The carefully researched attention to details are also getting less and less attention - Someone makes a point of being called 'Ms' without characters reacting to how uncommon that was at the time. Screw top wine bottles are mentioned like they were commonly in use at the time. Computers appear that haven't even been made yet (TRS-80 Model 4). Get ready for more obvious product placement too.
It's symptomatic of the changes in the whole show. The decline is subtle enough that it's not become unwatchable, it's just much less enjoyable. What was once one of the more interesting shows to watch is now heading more toward something you put on in the background. The interviews with criminals are still interesting, but that's about it.