Neither by default. I look to be convinced that they've implemented containers because it made sense for their technical architecture or strategy.

I have a vendor who pitched their new "cloud-native" re-platforming project and it really spooked me. It's data management-type tool that was migrating from a traditional on-prem client/server architecture to an AWS-hosted Angular interface with a MongoDB backend. I got the same pitch a year later and the entire stack had changed. I was spooked the first time; now I'm really spooked and thinking about migrating off the platform.

The key thing is that it's a sign that you're effectively going to be outsourcing a complete userland to them, which means you'll be much more dependent on the vendor for security updates to anything their code depends on as well as their code itself.

Whether or not this is a good idea depends on your situation and their level of competence.