How important is it to stick to the platform guidelines in mobile app design?

I'm designing a mobile app.

The developer is using a framework which uses native building blocks for iOS and Android. Customising means extra dev work.

Developer says: "Why waste time when guidelines exist that have already been researched? Let's use native building blocks."

Example: tabs at the bottom for iOS, top navigation for Android with next button and hamburger menu.

She has convinced me, but, at the same time I realise that I don't know enough to be satisfied that she's right. I see millions of apps with a vast variety of combinations out there. Some of these have excellent UX, despite not strictly adhering to guidelines.

I am torn between the thought that she's using UX as a motivation when what she wants is an easier life developing, but on the other side I see her point.

Should I stick to the design guidelines?