The obvious advantage is that you don't need to deploy your root cert on client devices, or manage your own PKI. Reissuing certificates shouldn't be a problem

The obvious advantage is that you don't need to deploy your root cert on client devices, or manage your own PKI. Reissuing certificates shouldn't be a problem since Let's Encrypt has a rate limit exception for renewals, though if I were managing 65,000 servers I'd be a little hesitant to put that kind of burden on Let's Encrypt's infrastructure without contacting them first, just as a matter of courtesy.