TBC heroics are about as hard as low Mythic+ keys, like level 3-5 or so.
There are still plenty of things that can wipe you if you don't handle them properly, and trash mobs hit like fucking trucks, so CC will be necessary until your tank/healer overgear them.
Also, some heroics are way harder than others. Shattered Halls, Shadow Labyrinth, and all of the Tempest Keep heroics are noticeably harder than the others.