There are a few potential issues I think, and really a lot depends on your preferences.
- Bulk - anything that fits a 3 year old is going to be pretty big on a newborn. So newborn clothes may not fit and some people are concerned that the baby's bum is raised so much when they're laying down because of the bulky diaper.
- Will it fit to potty training? Some one-size diapers do, some don't, and this one depends on your kiddo: how big they get and how soon they potty train. Because of this and the previous point many people end up buying newborn, then one size, then extra large size diapers, which ends up being 3 different sizes not one. Some diapers are better at fitting newbies, others will fit much larger kids - depends on the brand.
- How long will the diaper last? Are you planning to use them on future kids? Many one size diapers will not last through multiple kids, which really is not that surprising since they'll be washed every other day for 2-3 years non stop, right? Having a large rotation of diapers will help with this, but if you spend more on a large stash you may find that it's not a big difference from getting a smaller stash of a couple of sizes and getting a more trim fit. Some of the cheap (meaning cheaply made, not inexpensive) diapers out there will not even last all the way through one kid before they wear out.
Personally, I'm totally into cloth diapering. But DH is not that excited about it so I don't want the bulk at the beginning to put him off before we even get started. So I figure sized diapers may be a good investment. Plus there is quite a market out there for used diapers believe it or not, so if they stay in good condition you can resell them and get some of your money back. I actually bought some of ours used and if you can do that it may not be more expensive at all than going with one size.
And another strategy, which I've heard some use, is to invest in just a few sized diapers to go nicely in those tiny outfits or for photos or whatever and use a more bulky system the rest of the time.
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