I believe it largely depends on the business domain. I worked for many years in fintech, for banks and insurances.
Consider the following examples.
Insurance: Given 10 input fields about a potential customer, produce a quotation and an illustration for a life insurance policy. At the UI level you create a form or a set of pages to collect birthdate, gender, job, hobbies, smoker/non smoker and so on. At the back-end level the above reflects in countless queries against trivial decision tables, and a lot of business logic and maths to be applied for each year/month of the policy duration in order to create a projection of the insurace coverage, taxation and many many more. The front-end will eventually display a nice table with all the numbers.
Fintech, financial planning. Collect your client needs (buy a house in 10 years, buy a car in 5, pay college for kids), propose an investment plan that starts with some initial investments in specific investment products with monthly installments, add some occasional big investments, project investments growth over the years using different mathematical and statistical formulas, produce a numeric data series that the front-end will eventually use to draw a plot chart.
In both above examples front-end work is way less complex than back-end. Technology could also imply complexity: at the back-end you may have to learn Java, Spring, Database, RESTful, some queue API (Kafka), big data for reports (Hadoop, Spark) and so on. All this complexity is hidden to the front-end developer.
On the other hand, if you are building a blogging platform, probably the complexity is the same at the back-end and the front-end or is higher at the front-end.
So in my opinion there is no correct answer if you don't take into account the business domain. Having worked in fintech, insurance, document management, few more in my career, in all these domains in my experience back-end was more difficult or at least not easier than front-end. Especially if you work in a product company, and every implementation needs to be carefully designed to provide stable API, design for changes, security, code optimised for large numbers of users (hardly a problem in front-end since JS is running on each user's browser).
Consider the following examples. Insurance: Given 10 input fields about a potential customer, produce a quotation and an illustration for a life insurance policy. At the UI level you create a form or a set of pages to collect birthdate, gender, job, hobbies, smoker/non smoker and so on. At the back-end level the above reflects in countless queries against trivial decision tables, and a lot of business logic and maths to be applied for each year/month of the policy duration in order to create a projection of the insurace coverage, taxation and many many more. The front-end will eventually display a nice table with all the numbers.
Fintech, financial planning. Collect your client needs (buy a house in 10 years, buy a car in 5, pay college for kids), propose an investment plan that starts with some initial investments in specific investment products with monthly installments, add some occasional big investments, project investments growth over the years using different mathematical and statistical formulas, produce a numeric data series that the front-end will eventually use to draw a plot chart.
In both above examples front-end work is way less complex than back-end. Technology could also imply complexity: at the back-end you may have to learn Java, Spring, Database, RESTful, some queue API (Kafka), big data for reports (Hadoop, Spark) and so on. All this complexity is hidden to the front-end developer.
On the other hand, if you are building a blogging platform, probably the complexity is the same at the back-end and the front-end or is higher at the front-end.
So in my opinion there is no correct answer if you don't take into account the business domain. Having worked in fintech, insurance, document management, few more in my career, in all these domains in my experience back-end was more difficult or at least not easier than front-end. Especially if you work in a product company, and every implementation needs to be carefully designed to provide stable API, design for changes, security, code optimised for large numbers of users (hardly a problem in front-end since JS is running on each user's browser).