Buy the boring. Build the edge.
Build the one thing that makes you different. Buy everything else. Most teams get this exactly backwards.
Almost every build-versus-buy decision comes down to one question that rarely gets asked out loud: is this the thing that makes us different, or is it plumbing? Get that right and the decision is easy. Get it wrong and you spend your best engineers on a commodity.
Buy the boring
Authentication, billing, email, storage, the standard CRM: none of it is your advantage, and all of it is solved. Building your own is a way to feel in control while pouring your scarcest resource into something a vendor will do better, cheaper, and with a support line. Boring problems are boring because they are already solved. Buy the solution.
Build the edge
There is usually one thing, sometimes two, that is genuinely yours: the model, the workflow, the piece of judgement your customers actually pay for. That is the thing you never outsource, never wrap around someone else's product, and never let a vendor own. If it is your differentiator, it is your build.
The expensive mistake runs both ways
Teams get this backwards in two directions. They build commodity infrastructure to feel in control, and they buy or bolt a generic tool onto the one place they should have built something distinctive. The first wastes money. The second gives away the moat. The second is the one that is hard to undo.
Customise only in the middle
Between the boring and the edge sits a middle band where the honest answer is buy and extend: take a solid product and shape it to fit, without pretending the whole thing is your invention. The skill is knowing which band you are in before you commit, not after.
Spend your build budget where you are trying to be the best in the world, and nowhere else. Buy the boring, build the edge, and be ruthless about which is which.