I've run about 3 Core scenarios and played 1 module, 3 levels of Emerald Spire and 6 odd scenarios. The highest level thing I've played or run was subtier 4-5
I'm finding Core distinctly harder (both as a GM and a player). Its certainly still winnable but its harder. One death so far but that was partly aggressive play and largely bad luck (crit rolling above average against a favoured enemy). The subtier 4-5 could have been a TPK but the GM was softballing a little. One scenario I ran would have been a TPK but I softballed a little.
Core is definitely starting to cause better play to occur in general. People are actually caring about tactics. Poor tactics can doom you.
There is definitely less room for mismatched parties, parties playing up, characters who don't pull their weight, players who don't pull their weight, etc. One or more problems here can really be a problem.
Going through a LOT more expendables. Especially CLW charges.
Almost all of the Core that I've experienced has been with experienced players, not new players. With that group, I like the effect.
But I expect lots of later season higher level scenarios will be quite deadly, especially with a GM who doesn't like to pull punches.