major obstacle
serious obstacle

In the short term, perhaps the most serious obstacle is a problem of deep theory.

In the years to come, he faces more serious obstacles.

This would seem to be a serious professional obstacle, but apparently not.

The official speaking today said such a test would create "serious obstacles to peace."

However, there are a few serious obstacles in the way of his return.

For now, the most serious obstacle to the project appears to be political.

A serious obstacle was the lack of funds with which to buy arms.

This difference is a serious obstacle in building a common identity.

It is taking some time, but I do not see any serious obstacles.

Vines can be a serious obstacle even for those who simply want to get past them.