What this seems to be saying is that serverless is great if you are doing javascript-heavy SPA's, otherwise not so much. Serverless is good if you don't actually need to talk to the server.
I build huge serverless backend applications, and IMO it's been fantastic. There is a learning curve, because your application's execution environment is pretty different from traditional applications, but it's allowed us to build a remarkably complex and scalable application, very quickly. And it's been pretty maintainable too.
Basically we have an API that performs asynchronous data analysis and processing. Our fronting service receives a request, writes some metadata, and places the request in a "queue", which is picked up by a backend poller that starts a workflow execution, which orchestrates the fulfillment of the request. This is all serverless (AWS tech...API Gateway, Lambda, Step Functions, S3, DynamoDB, CloudFormation, CloudWatch, etc.). Serverless makes deployments much easier, since we can version our Lambdas and State Machines using CloudFormation, and have many different versions running at the same time (not fun if you're managing your own hardware!). We have a CD pipeline that builds code changes, deploys them to a test account, and runs integration tests. We use CloudWatch Alarms to monitor production and alert us of any issues.
We have some development scripts for pushing code changes with CloudFormation for testing during development. We use that to develop, then once we check the code in, it works its way through our pipeline and into production.
It turned out really well and am very happy with it thus far. I expect maintenance to be minimal thanks to serverless.
Though I don’t think it’d work as well for an app with 100+ database tables. Definitely a middle ground in there.
I work with a platform that also does CRM/POS/ERP (plus a bunch more), and the products alone have over 10 tables for describing them: the base products, their variants, a list of generic variant attributes, a table detailing which variants have which attributes, a table for specifying the values that those variants have of those attributes, the product categories, two tables for configuring the taxes applied to each product (on purchases and sales), the product images, the list of suppliers.
We're already on 10 and we haven't even used those products for anything (stocking, selling, invoicing, purchasing, etc, etc).