I looked at KK and NH when I bought my Selene 57. To be honest, all 3 would have done the trick, but I'm very satisfied with my Selene. I was looking for a single engine, full displacement long distance trawler. Here is my impression of our Selene after 2.5 years of ownership and about 10K miles traveled from Maine to the Bahamas, living aboard nearly full time and anchoring out almost exclusively.
The layout of the 57 is terrific. It's very similar to a Nordhavn 57, which I consider to be their best boat. 3 stateroom, with midship master and spacious VIP stateroom. It has a fairly low profile, a very well protected prop with large keel that makes it track very well, everything is easy to access (relatively), it is very easy to move around in, handles different sea states very well, is relatively simple and intuitive, has a good selection of high quality components, has amazing woodwork inside (much better than N or KK), has good speed and fuel economy, has good access in the engine room, has a huge lazarette with large freezer, and doesn't consume a huge amount of electricity to operate. It's weight (according to the Selene website) is 102,000 lbs, but I think it is slightly heavier, maybe 110K. And the boat look awesome. One of the best looking boats in the water IMO. We get lots of comments about how good she looks. In comparison the N57 is 122K pounds and 6'-2" draft (according to N website).
The downsides are: the gelcoat is not good. My boat has visible defects in parts of the hull that can be seen from 10-15 feet away. The topside gelcoat is thin in spots, but that may be from prior owner's care. The draft is 6'-6" (the website says 5'-9", but I measured it when it was on pavement and the website is wrong). I need to stoop in the engine room, not stand up. The ER is well laid out and easy to get around, but my back starts to bother me after a while. I do hear creaks when we're in heavy seas that are may be common with other boats, I'm not sure. It may mean Selene needs to isolate the woodwork from the hull a little better.
To answer the OPs question, I think Selene's biggest achilles heel is their gelcoat process. I don't think it's isolated to early Selene's only. I bid on a Selene 59 a few years ago that was in Seattle that had a gelcoat issue. I think that was a 2018 model. It got a lot of interest here on TF. I do believe this is a cosmetic issue only, and not all Selenes have gelcoat issues. So I don't think Selene had good control of the gelcoat process. Maybe they do now, dunno.
My perception of Nordhavn is that their boat designs are all over the map. The 57 is wildly different that the 55. They have a lot of stairs to climb, making movement through the boat slower. The hulls are beefy, and the engine rooms are nicely done.
KKs seem a little dated with the parquet floors and big, turned wood column supporting the galley cabinets. I'm not a big fan of the master stateroom in the bow, and the upward sloping deck. They are efficient through the water, but also a little slower.
Anyway, that's my impression. Yours may be different.