難しい and 厳しい: difficult tasks and strict standards
Distinguish a difficult task from strict rules, and examine 厳しい with weather and schedules.
Difficulty in solving something
この問題は難しくて、一人では解けません。 says that the problem is difficult and the speaker cannot solve it alone. 難しい, read むずかしい, evaluates difficulty in solving it.
明日までに全部終えるのは難しいです。 says that finishing everything by tomorrow is difficult. The difficulty may arise from time or conditions, not necessarily complex content.
Apply a strict standard
この先生は時間に厳しいです。 says that this teacher is strict about punctuality. に with 厳しい, read きびしい, marks the standard or area treated strictly.
規則は厳しいですが、内容は難しくありません。 says that the rules are strict but not hard to understand. Separate demanding requirements from difficulty of comprehension.
Harsh conditions and tight schedules
今年の冬は寒さが厳しいです。 says that the cold is severe this winter. 厳しい can describe harsh conditions without anyone enforcing a rule.
この日程では厳しいので、締め切りを延ばせますか。 asks whether the deadline can be extended because the schedule is too demanding. 厳しい is not limited to moral strictness.
Is 規則は厳しいですが内容は難しくありません contradictory?
Show the answer and why
No. The rules are strict, but their content is not difficult to understand. Strictness of enforcement differs from difficulty of understanding.