1) what is wrong with "Some people are in favour of..." ?
I read a sample sentence from dictionary "Not many people are in favour of using..."
There is nothing wrong with both. They are both grammatically correct.
2) Yes, you are right, I should use modern cities and not countries.
But just curious that will I lose marks in the exam ?
Well... they follow a marking scheme and I hope you would get an idea about how you will be marked if you visit the following website;
ielts.org/PDF/UOBDs_WritingT2.pdf
3) I know that I'm quite poor in grammar.
Do this essay has any obvious grammatical errors ?
Ok, there are a few I too find in this essay. They may be careless mistakes and overall, this essay is not one with bad grammar :) However, pay attention to not making any grammar mistake because it is something serious when it comes to scoring :)