My friend tried bleaching her hair even though it was already pretty fried from previous dye jobs. She followed all the advice online, used a good conditioner, and took her time. But her hair ended up feeling like straw and breaking off in chunks. It was so bad she had to cut several inches off to save it. She learned that sometimes damaged hair just can't handle more bleach, no matter how careful you are. Now she sticks to deep conditioning treatments and avoids any more chemical processes.
My cousin's hair was a total wreck from box dye, but her stylist used Olaplex 1 and 2 in the bleach mix and did it in two sessions a week apart. It lifted perfectly and felt better after. The right bond builder and technique can save hair you'd think was a lost cause. I've seen it work on level 10 damaged hair that still took platinum.