It's always tough ramping up a new development team. Unless you've actually worked with them in the past, I would consider any team "unproven" and assume they must prove their credibility.
The next question is how to vet an unproven development team. First thing is make it clear they don't have a contract for the whole app along with a bag of money to build it. Break up the project into distinct modules that you can track and pay for over time, as they prove themselves. Make it clear that you are only going to commit to paying for the first module and if that goes well, they can bid on the coming modules.
If a team knows they have solid skills and can deliver in this model, they shouldn't have a problem with it. If you have a team that is just trying to rip you off, they will push back on this and that should be a red flag. Also having someone able to do a code review is good to let you know if their code is bad, or if it's conforming to best practices to some degree. An app can work and be a nightmare to maintain, but that's not great for maintainability.
You are in control of the timelines and it is their responsiblity to prove themselves by meeting that timeline. If they see a problem with deadlines, or issues arise it is of course a conversation that can be had. If they try to push the timeline out too far for a given module, break that module into 2 or more modules with an acceptable timeline.
Then you're only on the hook to pay for a single module at a time. This should be an amount that you're willing to lose if things go south. I know that sounds bad, especially to investors, but it's really the only way to mitigate the risks of dealing with a company that you don't know personally.
Hope this helps.