The most crippling limit placed on Basic Members is the ability to only download detailed cache information for up to 3 caches per day!!! Additionally, searches will only return Traditional type caches. Summary information for additional caches (up to the limits described below) and all trackables and logging functionality are fully available to Basic Members.
If you are a casual cacher, finding only a few caches per week, a Basic geocaching membership may be all that you need; however, to take full advantage of the API's instant access to detailed cache data, a Premium membership is recommended.
Please note: There are no added limitations within Neongeo based on your geocaching.com membership level. You have full access to all parts/features of the app; however, some functions may not be useful to Basic members who do not have access to geocaching.com Premium Member features (e.g. Pocket Queries).
There are additional limits placed by Groundspeak for all API users, although with normal use they are not likely to become a problem. These have not been confirmed from an official source, but have been gathered from various forum threads: