Simon R. Green's work has long been my guilty pleasure of choice as literature goes. The man's a master with witty turns of phrase and his worldbuilding is downright addictive. Perhaps most importantly, he enjoys what he does and it shows in the way he writes--I can go on at length about the technical merit of a Kelly Link, Ted Chiang, or even Green's most frequent comparison, Jim Butcher, and I adore all their work quite a bit. But none of them brings Green's sheer enthusiasm to the page. It's one of the reasons I suspect the Nightside series, in particular, is going to end up aging better than the Dresden Files, even if it takes a long, long while before it hits a similar level of popularity (if it ever does).
Night Fall is odd in that it's missing some of that enthusiasm. The phrases are still mostly great, the worldbuilding is still mostly top notch, but there's a sense that Green is tired of this shared 'verse he's built and wants to do something new. So he's burning down the set, blowing up the building in which the set resides, and then calling in an artillery strike just to be really sure about it. The end result is a wild, fun ride--especially when it focuses more on the Nightside portion of the cast--but it's also got a sense of fatigued finality to it. It especially shows in the signature Green Twist, which I'd define as "a Deus ex Machina that you don't mind because it's fun, dammit." The fun's just missing sometimes, and most often at key moments when its presence can make or break a scene for you.
More than most of his work, Night Fall is a crossover. It struggles a bit, I think, because it's more Secret Histories than Nightside. The Droods just aren't all that interesting compared to the glitz, glitter, glory, and godawfulness of the Nightside. They make for extremely effective antagonists, especially as the Nightside struggles to overcome their armour, but the story as a whole would benefit from having less of their perspective for that very reason. Moreover, Eddie and Molly mostly just get in the way, while John and Suzy don't really do all that much in the grand scheme of things.
And there's no real comeuppance for any of the war's architects (direct or indirect). That got me the most. It kinda felt like Green forgot about some of his earlier writing, shrugged when someone brought it up, and just kept going because he wanted to be done already. The epilogue in particular feels like he sat down after the final page, rubbed his hands together, and said, "Screw it, here's one more curveball. TIME TO THROW TOGETHER THE ACKNOWLEDGMENTS AND DO SOMETHING ELSE!"
Although his best twist is the one that closes out the issue of what's affecting the Nightside's borders. Looking at the timeline of the book, then looking at the politics of the real world UK during both its writing and publication--Green saw Brexit and decided to have a little purely implicit fun with it. Nobody else could or would do that, and nobody else could get away with it the way Green does (if you're even keyed in enough to notice it). No one can convince me otherwise, except by arguing that it was a brilliant accident, and even then I'd second guess them.
Without diving too deep into it or giving away any major spoilers: A whole freaking lot of characters die off in Night Fall, including some fan favorites. Some of these deaths are satisfying, some are intensely problematic, just about all of them make sense, and most are quick, brutal, and undignified--which is fine, but a few of the last ones in particular would've benefited from slowing down a bit and making it more of a struggle. I'd also argue it's a tie for best death in the book: there's a long, drawn-out redemption that works beautifully and gives us a visit from an old favorite, and there's a short emotional roller coaster of two minor characters dying off that perfectly encapsulates all of Green's writing. The short one takes up less than half of page 360, the long one spans pages 342-347.
One last thing, and I admit it's petty as Hell, but there's a ton of grammatical stuff in Night Fall that had me side-eyeing the copyeditor. Some of it works because Green's so damned good with his phrasing, but some of it just looks like the editor fell asleep at the wheel. I'm not normally one to call out an editor, especially since I'd like to be one and authors are finicky beasts that can and do ignore your advice at their peril, but it really does detract from the reading at certain points.