t2's are great until something happens and you really need the full CPU, then you get throttled and suddenly your service goes down because it can't keep up with the load.
If you use t2's for anything important, keep an eye on the CPU credit balance.
Just applying selinux policy on CentOS 7 will kill the CPU credits on a micro instance. Running updates is a risky businesss.
But if you get throttled in the first place that means you're going to have some kind of performance degradation since your server was using more than the baseline level. Web apps being IO bound isn't relevant here, because the only requirement for issues to arise is for your server to have consistent 20%+ cpu usage.
Well, they're relevant in that a heavily IO bound app is probably unlikely to use much CPU -- it's too busy waiting on IO to use 90% of CPU, and maybe does stay mostly under 20%. Obviously this depends on a lot of details beyond "IO-bound", but it is not implausible, and I think does accurately describe many rails apps.