There are zero similarities between Brown and Sumlin, it's premature to even attempt to compare the 2.
Mack coached for 29 years as a HC, Tulane, UNC and the final 15 at Texas where he found most of his success. Mack was hired in 98 and won his fist South division title in '99. He won 6 South Division titles, 2 Big 12 titles and 1 National championship with an overall record of 238-117-1 and 21 Bowls appearances going 13-8... Mack had twelve 10 win or more seasons with twenty seasons at 8 wins or more.
Sumlin has been a HC for 8 years with limited success thus far in a weak C-USA where he won 2 West titles, O conference titles before jumping ship for TAMU where he has had 1 great season in his first season as the HC, 0 division titles and 0 conference titles with an overall record of 71-33 with 6 bowl appearances in 8 years going 4-2 in those games. Sumlin has a SEC conference record of 17-15 beating 4 scrubs a year based on how their seasons finished or being FBS to achieve an overall record of 36-16 while coaching in the SEC. Sumlin has had three 10 win or more seasons with seven at 8 or more. He's already sitting on the hot seat in Aggieland and you wan't to compare him to Mack Brown?
You would be a bloomin' idiot to attempt to compare the 2.
Try comparing Sumlin to a guy like Dabo Swinney 75-27, 5-4 in bowls, 2 ACC titles, 4 ACC Atlantic titles! Both have coached for 8 years.
Hell, Charlie Strong has 2 Big East Championships with two 10 plus win season in 6 years as a HC going 48/29 overall and 3/2 in bowl games for teams he coached for the entire year. Sumlin behind in titles?
What's funny is Tony Levine won the ticket City bowl with those Briles recruits he left for Kevin the year Sumlin left with 12 wins before going...5-7, 8-5 and 7-5 in the following three seasons with what Sumlin left him in a lesser conference than Sumlin coached in as they switched to the American.