Haskell is a problem _for me_ simply because I don't know it :-)
Seriously, though, I was thinking about the popularity of the language the ERP is written in and the ease of finding a developer to customize and maintain it.
This is probably a moot point, though, as the software publisher would probably be glad to receive money for this type of work :)
Text files don't seem to me like the best storage format for accounting data. RDBMS with ACID compliance where written for a reason, weren't they ? :)