This is an interesting question. In terms of limitations, if you had a large enough network the OSPF database would become bloated, especially with Type 5 & Type 7 LSA's - essentially in duplicate due to the conversion that has to take place for the NSSA area. Maintaining such a Database could strain the processor on the device.
I would also say this would offer the potential for being a single point of failure within the network as if this device fails you will loose connectivity not only to Area 5 but to the external destinations too. This, of course, depends on whether this is the only router with this capablity. Other issues may arise if you were redistributing routes in to the external routing protocol in terms of filtering and again holding for example, an EIGRP topology table.
I'd be interested to hear other opinions on this question.