Below are few Challenges you may face during your project execution time
-It may be power planning- because you found more IR drop
-It may be low power target-because you had more dynamic and leakage power
-It may be macro placement-because it had more connection with standard cells or macros
-It may be CTS-because you needed to handle multiple clocks and clock domain crossings
-It may be timing-because sizing cells in ECO flow is not meeting timing
-It may be library preparation-because you found some inconsistancy in libraries.
-It may be DRC-because you faced thousands of voilations