|Probably true to a certain extent. The high amount of synthetic in pockets means that build-up happens easier and the consequences are worse. But fitteds and prefolds can still get stinky build-up, it's just that it takes longer and they're not likely to repel because of it.
The soap nuts have been great for me. They do work best in hot water and the first use is always the 'strongest' so to speak. You can use them up to about 2-4 times total (probably depending how hot your water is and how many nuts you're using). You can usually see them starting to get lighter and a lot mushier/falling apart when they're starting to be finished.
Some do use only 'fresh' nuts on diapers and then re-use the used ones for clothing. You can also add a little washing soda (a tablespoon or two is plenty) to up the cleaning power a bit and add a very ligh fresh scent. The lack of any scent at all does throw some people off I think.
Some also make (or buy) soap nut liquid by boiling the nuts in water for a little while. I don't have a real recipe off hand but it's something like a few cups of water and 3-4 nuts for about 5 minutes. Then you wouldn't have to worry about figuring out when they're done or not, you'd just follow the recipe and use the recomended amount of liquid. I know some places even sell soap nut liquid, that would be the absolute easiest and surest (even if not the cheapest) thing.
And for what it's worth, if Tide works for you (maybe the 'free' version for your little ones' sensitivities?) then there's nothing wrong with that. You can always use a more eco-friendly detergent for clothing and such. Better to use a less friendly detergent with cloth rather than have to constantly battle stink, rashes or the temptation to quit.