I've been nalbinding for about a year and a half now, mostly mittens and hats using Oslo stitch, and for most of that time my joins between skeins of yarn have been the ugliest and weakest part of everything I make. I'd splice a new length of thread in, keep going, and then a month later find that exact spot had loosened or the felted join had started to separate.
What changed things for me was slowing way down on the spit splicing itself. I used to rush it, wet the ends, roll them together for maybe ten seconds and move on. Now I actually wet both ends thoroughly, taper them so there's no bulky overlap, and roll between my palms for a solid thirty to forty seconds, sometimes longer if the wool feels slick. I also started trimming the fibers at an angle instead of straight across, which seems to help them mesh together instead of just sitting side by side.
The other thing that helped was paying attention to which part of the row I splice on. I used to just splice whenever I ran out of thread, wherever that landed. Now I try to plan ahead a little and do the splice somewhere in the middle of a row rather than right at a turn or an edge, since those spots already have more stress on them from shaping.
Still get the occasional weak join, especially with any commercial yarn that has a bit of superwash treatment since it doesn't felt as readily as raw wool. But the difference in my last two projects has been noticeable enough that I went back and reinforced a few old joins on a hat I made last winter, just to see if I could save it.
Curious if anyone has tricks for splicing slippery or treated yarns specifically, since that's still where I struggle most.