have difficulty or difficulties

[joham]

Michael Swan says in his PRACTICAL ENGLISH USAGE:
I'm having difficulty with my daughter's maths homework.
You won't have much difficulty (in) getting to know people in Italy.

Swan also says we mustn't use difficulties in this structure.

But I read these sentences in CAMBRIDGE LEARNER'S DICTIONARY:
People are having difficulties keeping up the payments on their loans. (keep)
They had difficulties finding somewhere to live. (somewhere)

Could you please tell me if we can use this structure if we like?

[Tdol]

The plural examples refer to repeated problems, Swan's examples refer to single ones. Also, the plural implies something more serious to me.

[joham]

Dear sir,

Thank you very much. When we say high winds, strong winds, floods, heavy rains, and even apologies, congratulations, respects, regards, they seem to refer to something of a greater degree. Am I right in thinking this way?

Thank you again!

[Tdol]

greater degree, repetition, duration etc