So what you have is a curve of diminishing returns. If you spend 2 hours on a legal problem, you might get an incomplete result. If you spend 10 hours, you might get something that meets the standards of good representation but still leave a few avenues unexplored. If you spend 20 hours, you might be able to explore those avenues and assure yourself that there are no risks lying there. If you spend 30 yours, you might just be gouging on tangentially related issues.

But you have a classic agency problem. It's in the lawyers interest to spend 20 hours, both because it increases hours and because that minimizes his chance of being wrong. It's probably in the client's interest to go for the 10 hour solution, because whatever risk is created by those unexplored avenues is likely outweighed by the extra time and cost.

Importantly, the threshold of risk is lower for the lawyer than for the client. Say one of those unexplored avenues requires delaying some deal by 2 days. That delay may be monetarily inconsequential to the client but is stinging for the lawyer because has advice has been found wanting, and that jeopardizes the client relationship.