So when you hire a dev unfamiliar with your stack you risk having all that (bad engineering in genetral) plus all the possible mistakes they can do with the spe

So when you hire a dev unfamiliar with your stack you risk having all that (bad engineering in genetral) plus all the possible mistakes they can do with the specific stack. Having good seniors on the team can mitigate this but honestly having people familar with your stack is a huge pro.