This is my third Ngaio Marsh novel and I still have somewhat mixed feelings. I’m not into her detective character at all — there’s been too little personality and depth, just a lot of surface shine — and the structure is now formulaic. Set-up for a murder with many potential motives -> murder which is very awkward for lots of people -> Alleyn investigates without explaining much to anyone -> Alleyn has a reconstruction done -> this flushes out the murderer, who incriminates himself without need for a trial, and who is the least suspected person -> an epilogue in which Alleyn explains everything.
I have got the next three books now, though. There’s something relaxing and easy about these, even a little compulsive, perhaps because I don’t care much for the characters and so for me, there are no high stakes. Generally the plots are full of coincidence, misdirection, and meta-nods at the genre (“if this were a murder story, you would suspect the least obvious one, of course!”).
I think you could pretty much class these as cozy mysteries.