ERA is going to be a flawed stat regardless of whether you're a starter or reliever. A starter's ERA can often be affected by circumstances and managerial decisions. For a recent example you have Pivetta's last start. He had given up 1 run through 5 and the Sox were up 10-1. Cora elected to squeeze another inning out of him, trying to save the bullpen an inning in a blowout. Pivetta gave up 3 in the 6th.
If Cora had pulled him after 5, Pivetta would now have an ERA of 0.90. (1 run in 10 innings.) Instead he has an ERA of 3.27. (4 runs in 11 innings.)