Update!!! I have used these for 4 washes (1 whites, lights, darks and a nappy wash) and i have to say i am impressed so far!!
With the clothes they went on a 40c 1hour wash with the balls all came out clean and smelling fresh. Nappies, i did a rinse with extra water (which i usually do). Then put the balls in and did our usual 60c wash with extra water and and extra rinse and then this morning just for good measure i gave them another rinse. (Now apparently you dont need to rinse with Ecoballs as they dont create build up but i was a bit skeptical and also i wanted to try and get rid of the build up of powder in them.)
Took the nappies out, inspected EVERY piece, especially the nappies i remember DD pooing in, smelt them, stretched the leg elastic to look between the 'folds' and everything. CLEAN

All smelt fresh. They are in dryer at the moment (which with the heat is usually where it honks of powder build up etc and so far no smell

Will see how they smell after DD has pee'd in them as of late they have been eye watering.
Note: We dont rinse nappies in loo if its a pooey one, just get the worst off with a wipe. We dont wet pail, dry only. Nappies had been left max 2 days. No essential oil was used
So, so far so good