Bruschi also only had 4 excellent "big play" years in his career, and 2 of those years were accompanied by low tackle numbers, which is a large part of why it took time for him to get that national attention. He only had 1 interception in his first 5 seasons, but he also only had 2 seasons with more than 50 tackles during that time. Bruschi's career high was 81 tackles. Mayo's had 98, 70 and 114 in his first three seasons, meaning he's been 'better' than Bruschi (as far as tackle accumulation) ever was. For Bruschi, the true excellence was in a relatively small window.
Mayo, however, won DROY and is being listed in the league's top 100 players this year, so I wouldn't say that he's not gotten his due. He has problems in coverage, and he needs more help around him so that he can be the beneficiary of pressured opponents. Get those issues solved, and he'll move up higher on the list.