I have no problem with SRBs for unmanned launchers like the Atlas 5 or Delta 4, but on a manned vehicle it's a recipe for disaster. Although they're far safer on a vertical stack booster like SLS than the parallel setup on the Shuttle, when an SRB goes wrong it goes wrong very fast and very violently with no way to cut it off.
Back in the late 90s one of a Delta 2's solids had a tiny, unseen crack in the casing, and it blew the whole rocket to hell just seconds after liftoff.
With the liquid boosters, if one of them fails, you can cut it off. And if you've set up the fuel line plumbing to make it possible, re-route the failed booster's fuel supply to the other one for a longer burn to make up for the lost engine, instead of being absolutely forced into a launch abort. It happened a couple times on the Saturn V second stage, such as the center engine on the S-2 failing on Apollo 13 resulting in a longer burn of the remaining engines and also a longer S-4B burn. Can't do stuff like that with solids.