Scalability objections: can you scale to our volumes?
Respond with suggestions for a phased roll out. Take your backend expert in to the meeting and present credibility on why you can scale.
Company objections: will you be around for long? What if you get aquihired?
Reiterate vision behind why are doing your startup (hopefully the right reasons). Talk about your investors and the expectations/commitments made.
Control objections: "We are uncomfortable giving up control over this part of our tech stack"
Try to create analogies. Understand other parts of the stack that they have given up control over and why they feel comfortable with those. Feed those expectations back as contractual commitments that you are comfortable making.