Nested IF, CASE, DO, LOOP, SELECT, WHILE and PROVIDE statements is a key ingredient for making what's known as "Spaghetti code".
Such code is hard to read, refactor and therefore maintain.
With the default threshold of 3:
IF param1 = 2.
IF param2 = 4.
DO 3 TIMES. "Compliant - depth = 3, not exceeding the limit
IF sy-index = 2. "Noncompliant - depth = 4