Oracle Text will not work with NVARCHAR2. What else might be unavailable?

If you have anything close to a choice, use a Unicode character set for the entire database. Life in general is just blindingly easier that way.

• There are plenty of third party utilities and libraries that simply don't support NCHAR/ NVARCHAR2 columns or that don't make working with NCHAR/ NVARCHAR2 columns pleasant. It's extremely annoying, for example, when your shiny new reporting tool can't report on your NVARCHAR2 data.
• For custom applications, working with NCHAR/ NVARCHAR2 columns requires jumping through some hoops that working with CHAR/ VARCHAR2 Unicode encoded columns does not. In JDBC code, for example, you'd constantly be calling the Statement.setFormOfUse method. Other languages and frameworks will have other gotchas; some will be relatively well documented and minor others will be relatively obscure.
• Many built-in packages will only accept (or return) a VARCHAR2 rather than a NVARCHAR2. You'll still be able to call them because of implicit conversion but you may end up with character set conversion issues.
• In general, being able to avoid character set conversion issues within the database and relegating those issues to the edge where the database is actually sending or receiving data from a client makes the job of developing an application much easier. It's enough work to debug character set conversion issues that result from network transmission-- figuring out that some data got corrupted when a stored procedure concatenated data from a VARCHAR2 and a NVARCHAR2 and stored the result in a VARCHAR2 before it was sent over the network can be excruciating.

Oracle designed the NCHAR/ NVARCHAR2 data types for cases where you are trying to support legacy applications that don't support Unicode in the same database as new applications that are using Unicode and for cases where it is beneficial to store some Unicode data with a different encoding (i.e. you have a large amount of Japanese data that you would prefer to store using the UTF-16 encoding in a NVARCHAR2 rather than the UTF-8 encoding). If you are not in one of those two situations, and it doesn't sound like you are, I would avoid NCHAR/ NVARCHAR2 at all costs.

Responding to your followups

Our application is usually alone on the Oracle database and takes care of the data itself. Other software that connect to the database are limited to Toad, Tora or SQL developer.

What do you mean "takes care of the data itself"? I'm hoping you're not saying that you've configured your application to bypass Oracle's character set conversion routines and that you do all the character set conversion yourself.

I'm also assuming that you are using some sort of API/ library to access the database even if that is OCI. Have you looked into what changes you'll need to make to your application to support NCHAR/ NVARCHAR2 and whether the API you're using supports NCHAR/ NVARCHAR2? The fact that you're getting Unicode data in C++ doesn't actually indicate that you won't need to make (potentially significant) changes to support NCHAR/ NVARCHAR2 columns.

We also use SQL*Loader and SQL*Plus to communicate with the database for basic statements or to upgrade between versions of the product. We have not heard of any specific problem with all those software regarding NVARCHAR2.

Those applications all work with NCHAR/ NVARCHAR2. NCHAR/ NVARCHAR2 introduce some additional complexities into scripts particularly if you are trying to encode string constants that are not representable in the database character set. You can certainly work around the issues, though.

We are also not aware that database administrators among our customers would like to use other tools on the database that could not support data on NVARCHAR2 and we are not really concerned whether their tools might disrupt, after all they are skilled in their job and may find other tools if necessary.

While I'm sure that your customers can find alternate ways of working with your data, if your application doesn't play nicely with their enterprise reporting tool or their enterprise ETL tool or whatever desktop tools they happen to be experienced with, it's very likely that the customer will blame your application rather than their tools. It probably won't be a show stopper, but there is also no benefit to causing customers grief unnecessarily. That may not drive them to use a competitor's product, but it won't make them eager to embrace your product.

Could we also expect performance breakage if our application (that is compiled under Visual C++), that uses wchar_t to store UTF-16, has to perform encoding conversions on all processed data?

I'm not sure what "conversions" you're talking about. This may get back to my initial question about whether you're stating that you are bypassing Oracle's NLS layer to do character set conversion on your own.

My bottom line, though, is that I don't see any advantages to using NCHAR/ NVARCHAR2 given what you're describing. There are plenty of potential downsides to using them. Even if you can eliminate 99% of the downsides as irrelevant to your particular needs, however, you're still facing a situation where at best it's a wash between the two approaches. Given that, I'd much rather go with the approach that maximizes flexibility going forward, and that's converting the entire database to Unicode (AL32UTF8 presumably) and just using that.