Warning: The following is entirely opinion. Not everyone agrees on all of these romances (hence the lengthy discussions!).
A: A’s romance is overwrought. It’s the RO you would expect from an really old Harlequin book, where hand touches were scandalous. It’s not just slow burn, it’s super duper, extra slow, slow burn. The first two books, you get some “looks” and an almost hand touch. And overwrought dialogue that results in A running away from you like a 13-year-old child with their first crush. If you’re patient, go for it. If you frustrate easily, look elsewhere.
Edit to A: Despite the slowness, I will bet a batch of brownies that A-mancers get a relationship before M-mancers do, so you can take that into consideration.
N: Sappy, schmappy, give-you-cavities-sweet romance. They call N “Suavewell” but I’ve never seen it. It’s just… too much for me. If you like super sweet, overly nice, wimpy ROs, this is the one for you. That is probably harsh, but I really don’t like N’s path. I had to put a hellion with him to make him more tolerable for me. But I don’t like overly sappy shit, so that’s just a personal preference. I do give N bonus points for loving dogs, though. I can’t dislike anyone who loves dogs.
F: Childlike romance. F is fun, and direct about what they want. They’re sweet without being overly sappy (sorry, N), funny, snarky, sometimes rude, and a bit on the lazy side. For that reason, I paired F with an equally unprofessional woman-child. It’s a fun romance, but it seems very, very young to me.
M: Sex. More sex. If you don’t want sex, M still sticks around hoping to get sex. There are moments of sweetness starting in b2 (in b1, M is basically a complete and utter dick who clearly only wants to get laid and flat out tells F that they can barely manage to get excited about that much with the MC). The sweet moments are perfect… not putridly sweet, but genuine care. There are a few of them in b2. However, this is a long haul romance and will take a while to get into an actual relationship rather than a fuckbuddy/wannabe-fuckbuddy casual thing with M being nasty and rude to remind the MC that M doesn’t want anything with them. From the author’s tumblr, it seems to me that it will likely be beginning of b5 or b6 before M actually has feelings and declares them (if they ever declare them). So you basically get a “kind of” relationship with M (acting like a couple on occasion) with M taking a step forward and two steps back for a very long time. Unlike A, at least you get something from M, though, even if it’s just physical and one or two nice moments per book until nearly the end of the series.
That’s my take on it. YMMV.