Ca Reactor
IMO, calcium reactors are too expensive up front, and have too many parts that can fail/malfunction. Of course, that doesn't mean they can't be done right (and they are very hands-off when they are), but there's a high price and more risk than some other maintenance systems.
Dosers
Thanks mostly to DIY reagents and inexpensive Chinese peristaltic pumps, dosing (two-part, balling, kalk, kalk+vinegar, etc) has become an extremely affordable option. Doesn't mean it dials itself in, nor does it mean it can't be screwed up royally, but it's inexpensive and has very few moving parts to fail/malfunction.
I've never been a big-budget reefer, so I actually still dose manually via DIY drippers. I have built my own peristaltic dosers in the past though, and it's quite inexpensive and very easy.
-Matt