The best and only way to use CERA effectively is to compare catchers on the same team, during the same season- pitcher by pitcher.
Since the sample sizes are usually unbalanced starter by starter, it's hard to make any claim based on that data alone. Even if you look at OPS against pitcher by pitcher, there are usually only a few pitchers who were caught by two different catchers for over 25 IP. (25 IP is still too small to definitively judge anyone, but if just about every pitcher does better with one guy over another, I think you can make some suppositions.
That being said, here are the 2015 numbers by catcher:
By OPS against (notice the wide disparity of PAs against between individual pitchers)
PA Pitcher OPS against
Swihart
448 Porcello .766
356 Miley .756
301 ERod .662
274 Wright .719
272 Owens. 726
268 Kelly .768
143 Ogando .795
123 Barnes .827
114 Breslow 1.000
109 Taz .665
109 Layne .720
101 R Ross .778
93 Masterson 1.076
Leon
377 Buchholz .627
182 Miley .738
120 Porcello .934
106 R Hill .410
74 Breslow.550
Hannigan
300 Kelly .732
293 Miley .722
196 ERod .803
177 Masterson .779
169 Porcello .736
104 Tazawa .876
92 Buch .817
92 Breslow .850
Nothing here jumps out to say Swihart was much worse than Leon or Hanigan. The 2015 to 2016 numbers don't make Swihart look too bad either, if you look at only the pitchers he caught a lot of in 2015 that were here in 2016 with other catchers.
2015 OPS Against with Swihart vs 2016 OPS against
2015 w/Swi Pitcher 2016 OPS against overall
.766 Porcello .635
.756 Miley to SEA
.662 ERod .728
.719 Wright 653
.726 Owens 1.007
.768 Kelly .828
.795 Ogando n/a
.827 Barnes .709
1.00 Breslow n/a
.665 Tazawa .730
.720 Layne .699
.778 Ross .624
1.08 Masterson n/a
Porcello, Wright, ERod, Kelly and Barnes were the 5 pitchers Swihart caught the most in 2015 AND were also on the 2016 team.
Here are their ERAs and CERAs in each season:
Porcello
2015: 4.92 (4.47 with Swihart, 4.15 with Hanigan, 7.86 with Leon)
2016: 3.15 (6.00 Swihart, 2.52, Leon, 3.45 Hanigan, 3.64 Vaz)
Wright
2015: 4.09 (4.24 Swihart, 2.25 Leon, 3.60 Hanigan)
2016: 3.33 (1.35 Swihart in 6.2 Innings, 2.72 Vaz, 2.90 Hanigan, 6.46 Leon)
ERod
2015: 3.85 (4.08 Swihart, 1.29 Leon in 7 Innings, 3.89 Hanigan)
2016: 4.71 (3.00 Hanigan, 4.50 Vaz, 4.90 Leon)
Kelly
2015: 4.85 (5.10 Swihart, 4.20 Hanigan)
2016: 4.18 (1.35 Leon in 7 IP, 4.50 Vaz, 6.97 Hanigan)
Barnes
2015: 5.44 (4.61 Swihart, 6.94 Hanigan in 12 I
2016: 4.05 (0.00 Holiday in 3 IP, 1.37 Vaz in 20 IP, 3.38 Swihart in 3 IP, 6.04
These numbers show Swihart in a more unfavorable light.
We also have to remember that Swihart was pressed into action quickly and had very little time with the pitchers he caught before catching them in live games.
Still, I trust management's judgements on these things, and if they feel like he should not catch much, I trust they have good reasons to think so.