Contrary to popular belief, Nephthys actually has its place in the meta. If you control your early game well (which is actually very possible) as well as build a solid deck around Nephthys, your winrate should be pretty consistent. The main aim until mid-late game should be to take as less face damage as possible. However, it does get outclassed by certain decks, deckwise not skill wise. If you are looking for a control shadowcraft deck I recommend Dark Alice, which follows the same principle but has a much more low risk (relatively) lower reward than Nephthys :)
To be honest, unless you start first with like the best hand classical nep is simply too slow. However, Kawy is a great spawn counter. In order to utilize it though you need to survive and at least contend board quarries earlier. Mordecai isn't that good atm either since most of the meta is too fast and/or have counter measures in form of banishment. there are two scenarios in which you can still use nep .
1 you run it like a hybrid of midrange/ control with Kawy which Is tricky due to card costs/curve issues.
2 You go with like heavy control hoping to survive till late/ in which case you usually win anyway against majority of meta decks.
Also I will add 3 Play it as a meme in tyrant.
Bottom line is arm it is just too slow and non reliable. Having said that I do believe it is possible to come up with something competitive but it simply seems like too much effort at least till nerfs and shift of meta.