It depends! Here are a few things I would be thinking about…
First and foremost, the budget. Are you paying this developer for their time? If so, normally a complex VST instrument done properly would require a sizable budget (5 figures and up depending on the spec).
If you have this type of budget and are paying them an hourly or daily rate, then it needs to be considered that there’s going to be a significant amount of time they’re going to be dedicating to learning to use JUCE. There’s a balance here because every project is going to have its share of unique challenges and need for research, so just because a person hasn’t developed product x doesn’t mean they aren’t capable.
Now, the problem here is that most people don’t have that type of budget. This means that agreements need to get more creative, and you can work out an agreement where you pay a reduced rate in exchange for a partnership. It sounds like there could be risk for both sides - the dev needs to learn JUCE and you need a product, and if you do a royalty split on the product, it’s a win for everyone.
Second is the communication. Do you get along with this person and do they understand and agree with your vision for the product?
Third is realistic expectations. Many people mistakenly believe that one audio developer can deliver a product from start to finish and that’s not the case. You have so many considerations depending on the product - digital signal processing (DSP), user interface (UI), user experience (UX), graphic design, product vision, security, architecture and so on. It’s RARE to find one person who has a great understanding of all the areas you’ll need to create a complete project. I’ve seen many people lean heavily on developers to understand what the customer wants or expects. I think that’s a lot to expect out of one person. Most times, building a successful plug-in of relative complexity is a multidisciplinary pursuit that involves a team.
I hope this helps!