The clearest case is anything involving security or complex edge cases, like handling passwords or card payments. Established services have handled the hard parts and the attacks you haven't thought of. Building your own is slower and far more likely to be unsafe.
Build custom where it's your actual differentiator, the thing no off-the-shelf tool does the way your business needs. Spend your effort there and buy or borrow the rest. Knowing which is which is a big part of building efficiently.
Updated July 2026
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