It depended on the iteration of Mind Sear, really. Some expansions loved, others hated. Tethering it to a tank in WotLK worked out fairly well, once you dotted things up. Criticisms of its mobility issues aren’t wrong. Legion and onward had a stacking negative effect on Mind Sear’s performance.
Mind Sear wasn’t initially envisioned as a talent, so any iteration that made it a talent misunderstands its function, much like many base (earned by level) abilities are misunderstood in talent form.
I don’t think Shadow Crash is bad, but it does need some tweaking. Honestly, the fact that it misses is less of an issue than the long period between recasts. Groups move so fast that, even if it does hit, it won’t be up again for the next group. More like every other group.
I don’t want Mind Sear back so much as I’d like Shadow Crash better thought out.