Can I trust the ISP's all-in-one modem/router?

I am used to using a modem from my ISP and my own router. I'm thinking about switching ISPs but the new ISP only offers an all-in-one modem/router. It's probably safe but I don't like other people's equipment on my network.
And since they are dumb enough and care little enough about my privacy to offer a smartphone app to administer their router, it really gives me pause.
What say you good people? Am I paranoid for no good reason?

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