I am a Texas resident (born here and also graduated from high school in Texas) and would like to point out that it's not just UT, every state school (the other big ones with a decent amount of competition are Texas Tech and Texas A&M) follows this policy. In actuality, it appears that because the demand is so high for UT they have had to lower the number to 6%: https://news.utexas.edu/key-issues/top-10-percent-law/
The primary flip side to the top 10% law is that urban centers have much higher levels of competition than rural ones. If you happen to go to high school in Nowhere, Texas, it's quite easy to go to UT since achieving that top 6% is a somewhat trivial endeavor. Whereas if you go to an urban high school in a population center such as Houston, Dallas, or San Antonio, you have to really apply yourself.
The primary flip side to the top 10% law is that urban centers have much higher levels of competition than rural ones. If you happen to go to high school in Nowhere, Texas, it's quite easy to go to UT since achieving that top 6% is a somewhat trivial endeavor. Whereas if you go to an urban high school in a population center such as Houston, Dallas, or San Antonio, you have to really apply yourself.