Question: * What's the limitation?
* Is there a specific volume of data each can handle regardless of disk space?
* When to use what assuming licensing is not a problem?

Answer: 
I'd say if you want to compare apples to apples, then it is MySQL vs SQL Express, vs Oracle Express.

Or if you have $, then it is the MySQL support license, MS-SQL Standard, vs whatever Oracle's cheapest offering is.

In my experience, once you choose a language, e.g. Php goes best with MySQL, then you've chosen your DB. Java goes well with Oracle. C# goes well with MSSQL.

Similarly, if you choose your OS, then unix flavors run MySQL or Oracle, but MSSQL is windows only. MySQL and Oracle work on both unix and windows of course.

[If you need to buy many machines, then not having to pay OS licenses for the server helps in scaling](https://blog.codinghorror.com/scaling-up-vs-scaling-out-hidden-costs/).
Perhaps one variable which people overlook in these cases is the availability of expert support. Okay, so currently there's an oversupply of people who can help you with db issues, efficiency issues, disaster recovery etc. However this may not be always the case in the future, and it's the applications you use it for that may be the defining issue. Are there people in your organization who have experience in one or more of the relevant databases? (as it happens I believe that someone's who's become proficient in say Oracle, can become fairly competent in Sql\*Server or Mysql in a fairly short space of time) You state it's going to be used for your financial systems - perhaps you really need input from a consultant who's worked on implementing and/or supporting financial systems - for example I understand that Sybase is popular in City type firms. Or perhaps there's an off-the-shelf package that utilises a preferred database? Try and define exactly what your system(s) needs to do first.