Every year, the Dallas Morning News' Rick Gosselin compiles his special teams rankings from that season, based on the results of 22 different categories. The ranking provides as accurate of a look as we have of just how a team's special teams unit did the previous year, so it's a very good resource.
The Colts have the best kicking duo in the NFL in Adam Vinatieri and Pat McAfee, but that doesn't necessarily translate into the league's best special teams unit - or close to it. In Gosselin's rankings, the Colts were 20th in the league this year with a combined 369 points (teams are given points for their placement in each category, with one point being the best and 32 points being the worst in each category). For comparison's sake, the Baltimore Ravens were ranked number one with a score of 231, while the San Diego Chargers were ranked last with a score of 490.5.
Last year, the Colts ranked seventh in the NFL in Gosselin's rankings with a score of 273.5, so they obviously saw a huge drop this year - with a 13-spot drop and a score 95.5 points higher. The 22 categories that he takes into account are as follows: kick returning, punt returning, kick coverage, punt coverage, starting point, opponent starting point, punting, net punting, punts inside the 20, opponent punting, opponent net punting, field goals, field goal percentage, opponent field goal percentage, extra point percentage, special teams points scored, special teams points allowed, blocked kicks, opponent blocked kicks, takeaways, giveaways, and penalties.
Adam Vinatieri was tremendous as usual in 2015, converting 25 of 27 field goals (92.6%) and ending the season with a streak of 25 made field goals in a row. Pat McAfee was also phenomenal, averaging 47.7 yards per punt and pinning 28 punts inside the 20 yard line without having a punt blocked and with just six touchbacks. McAfee also had a fantastic year as a kickoff specialist, so the problem with the special teams in 2015 wasn't with either of the kickers - it was with everyone else.
The Colts allowed three punt return touchdowns this year, which is tied for the most given up in a single season in franchise history. They allowed 10.7 yards per punt return and 19.5 yards per kick return. In addition to that, the Colts didn't score a single special teams touchdown, though they did get a nice boost out of Quan Bray late in the season in the return game. In short, the Colts' special teams unit wasn't the worst in the league this past season, but it also wasn't anywhere close to the best as the team saw a significant dropoff in their special teams play.