I'm not a pro enough to speak to your concerns but there are a number of large complex applications written in FMP that you may want to look at. Jungle Software is a good place to start.
The down side to FMP for me as a user of some of those apps is that they come with a stack of files. The runtime of a FMP application isn't packaged as a bundle so it can look a bit complex with a large app. We did some tests a long time back because FMP had a reputation of being slow. At that time (12 years ago) FMP needed to index the db or it was slow but once it was indexed it was as fast as anything else we tested. It's big upside for semi pros is that it is very easy to do basic stuff and end up with working tool. My experience with Access was extremely negative so I wouldn't compare it at all with FMP.
In the end it doesn't really mater what it was written in, if the software does what you want and is stable buy it. If it doesn't don't. It is very easy to get data in and out of FMP so the proprietaryness of the db format doesn't really enter into it.
