I use Nellie's and so far I love it. I use one scoop in my wash for 15 diapers, about 25 wipes, and 4 flats. I don't think it causes problems with build up because it isn't really soap. At least that is my understanding...I am a newbie though!
I love Nellie's. I was so impressed that only 1 tbsp (the recommended amount) cleaned our diapers that I started using it for all our laundry. I ordered the huge bucket from Costco online.
Currently I wash diapers EOD. So, about 16 of DS's NB dipes, plus 10 of DD's OS dipes. No issues with buildup or stink of any kind. I've been using it for 6+ months with no issues.
Re: Nellie's
I've used it twice and had one stinky diaper after they were done. I'm wondering if I holding use a tbsp and a quarter or a heaping tbsp or something.
Other than that I really really like it!