I think there are 4 buckets of data engineering problems, each with their own challenges and solutions.
Operational Data Engineering This is the detritus that grows like weeds as parts of other projects and often isn't recognized as a data engineering problem. We need to pull a file off an FTP server or hit an API and do something with it. Next thing you know, there are dozens of these little things that are not individually hard, but having visibility into dependency trees and failure cases becomes difficult because they are spread out everywhere and it's not obvious where to look when things go wrong. Tools like Apache Airflow are a good solution even if you don't use them in other ways because they can centralize monitoring, logging, and graphs. Scaling isn't resource intensive for these tasks because they are discrete. You can fan out. The scaling challenge for this type of data engineering is really about tending your garden and keeping things coherently organized.
Business Logic Data Engineering This is processing where the data is highly structured and sometimes even ordered or sequenced. It's hard to scale because you can't just throw things into a stream and apply multiple workers. You have to have a managed process and likely shared in-memory state that collects the worker results and applies strict rules to a process. This is the opposite problem from big data. It's small data, rigidly organized, and carefully managed.
Data Science Data Engineering This is sort of classic ETL with a twist. ETL systems are typically pretty static once the E, T, and L are known quantities. But working with Data Scientists requires that your pipelines have to be pretty flexible because scientists are doing experiments. But they also have to be repeatable and comparable, which means your pipeline has to maintain version. This is also the area where you are most likely to encounter Big Data, so you have to be prepared to change your mental model and be able to use tools like Hadoop and Spark to bring compute to where your data is.
Analytics Data Engineering This is classic ETL pipelines that move data from point A to data lakes or data warehouses. The key thing to understand here is what you are modeling at the endpoint. If it's a legit data warehouse, you are modeling business processes. If you aren't doing that, you are--by definition--pushing data to a lake. Understanding your endpoint is key to choosing your reporting and analytics tools to lay on top of your data source. Data lakes are a good use case for ad-hoc, SQL-driven reporting tools like MetaBase. But if you are sitting on top of a well-structured fact/dimension type of warehouse, you will want more formal tools like Tableau, Pentaho, or Cognos.