There are three things to keep in mind about the other question:
It was asked on Stack Overflow which has different question scope and guidelines than Programmers, so using it as an example of "but it was on-topic!" is invalid.
It was asked four and a half years ago, and site rules and guidelines change over time. While Stack Overflow has not had nearly as much change in this area as Programmers, it has still evolved. Pointing to any old question on the first few Stack Exchange sites (SO, Prog, SuperUser, ServerFault, etc) as evidence of being on-topic is suspect and should be approached carefully.
The question was closed anyway, several years ago.
Your question has two problems:
My qestion is why is it difficult for a developer who hasnt worked on Zend framework ?
Subjective. This is a question about the developer, not the technology. I have personally seen many cases where one developer easily picked up a new technology where another developer struggled.
Not only can we not answer because we do not know the developers involved, any answer would be isolated to those specific developers. The Q&A format is designed to be applicable to many people, not one or two: Stack Exchange is essentially a repository of questions and answers that people can find from Google and get an answer. What do I, random Internet searcher care, if person X had a hard time learning technology Y? I do not care, that does not apply to me.
And 2nd one : What tasks will i be working on as a Magento Developer?
Ask your supervisor, not the Internet. While someone may be able to say "Magento developers commonly perform tasks X, Y, and Z" perhaps your position is only X, or Z, or sweeping the floors because you got hired to allocate funds due to budget games (I have seen stuff like this happen, seriously).
We cannot predict the future any more or less than anyone else. Just like the weatherman, we might be able to make educated guesses but nobody knows for sure. That question is not objectively answerable.
I think your questions are answerable, just not in the Stack Exchange Q&A format. You are really looking for:
Tutorials. This will give you a taste of Magento to gauge its difficulty for you. Personally, I can read a tutorial and get a decent idea of how difficult a particular technology is. This may also help with "what tasks will I be working on?" because tutorials often cover the common tasks of a technology once they get past "hello world."
Blog posts. If you want to learn about a technology and all the subjective gooey bits that go along with it, read blogs on the topic. People will talk about their experiences, what they had a hard time with and why. They may make comments about "I was developing yet another X because that is what we do with technology Y" which helps answer your second question.
I think if you read good questions and answers on Stack Exchange and compare with good tutorials and blog posts, you will see that the formats are utterly different. Each resource has its strengths and weaknesses and they compliment each other.