From Longman Dictionary of Contemporary Englisha thorny question/problem/issue etca thorny question/problem/issue etcCOMPLICATEDa question etc that is complicated and difficult the thorny question of immigration policy → thorny
Examples from the Corpus
a thorny question/problem/issue etc• Melding the top managements also would be a thorny issue.• None of these struck me as particularly penetrating answers to a thorny problem.• In addition, sending encrypted data over international boundaries represents a thorny issue: it is still illegal in some countries. 