Are some languages harder than others?
It depends on many things; Chinese, for example, has no tense, but it has a writing system that is very different from the systems used in English. One thing might be harder, but another is simpler.
With English, I feel that it is an easy language to pick up at the start. Many students can start speaking it fairly quickly and progress rapidly through the early stages. this might be because it seems not to have the complexity and abundance of grammatical rules that many languages have.
However, this is also one of its difficulties; having mastered the basics of English, many students then find it hard to reach the very high levels, because there are so many exceptions to everything and so many funny little rules.
All in all, I'd say it is easy to begin with, but it gets a lot harder. if your goal is to reach fluency, then English is not an easy language. It has an enormous vocabulary, its spelling system is a mess and there are many tricky little rules.
There are other factors that affect it, such as how close the language is to your own. Spanish will be easier to a Portuguese speaker than a Japanese speaker because it is very similar.
Personally, I don't think any language is easy, not if you wish to master it. As languages embody entire cultures, they are highly complex things and require years of learning for most. I'm still learning a lot about English and I have been speaking it since I was a baby and teaching it since I finished studying it at university.