There are still two weaknesses:
1) Some configs are inherently global and cannot be phased. There's only one place to set them. E.g. if you run a webapp, this would be configs for the load balancer as opposed to configs for each webserver
2) Some configs have a cascading effect -- even though a config is applied to 1% of servers, it affects the other servers they interact with, and a bad thing spreads across the entire network
This is also why "it is always DNS". It's not that DNS itself is particularly unreliable, but rather that it is the one area where you can really screw up a whole system by running a single command, even if everything else is insanely redundant.
You can shard your service behind multiple names:
my-service-1.example.com
my-service-2.example.com
my-service-3.example.com …
Then you can create smoke tests which hit each phase of the DNS and if you start getting errors you stop the rollout of the service.
And the access controls of DNS services are often (but not always) not fine-grained enough to actually prevent someone from ignoring the procedure and changing every single subdomain at once.
It does help. For example, at my company we have two public endpoints:
company-staging.com company.com
We roll out changes to company-staging.com first and have smoke tests which hit that endpoint. If the smoketests fail we stop the rollout to company.com.
Users hit company.com
Then you can update the DNS configuration for company-staging.com, and if that doesn't break there's very little scope for the update to company.com to go differently.
The scope for it to go wrong is the differences in real-world and simulation.
It's a good thing to have, but not a replacement for the concept of staged rollout.
So if you've got some configuration that has a problem that only appears at the root-level domain, no amount of subdomain testing is going to catch it.