Why would you believe medications can do these things, even though you’ve no doubt heard of people whom pain medications don’t work on, and people for whom even the best cancer treatements failed?
You’re being very selective here and asking magick to jump through hoops you don’t expect the physical verifiable sciences to jump through.
Once again, that’s the thiung some people do on here - saying magick “has to be” X, Y and Z criteria you made up, or it can’t be real.
Try that same thing on a forum for musicians, car mechanics, or trainee medics and see how far it gets you! 
Has anyone seen healings such as an arm growing back? If not, is that type of thing unreasonable?
I should think so, because magick usually works to make the improbable, but possible, happen - for example it’s improbable that a tumour would cease growing, but NOT impossible, it’s improbable that male-pattern balding will reverse, but not impossible - those mechanisms DO exist.
But I’m not aware of any biological mechanism that exists for a limb to grow back in a human, so that human would have needed to leave behind low magick (which coerces probability) and be on the literally godlike level of altering manifest reality.
I think someone on that level could do it, but they’d have the problem of all the people who observed the person not having their arm, and honestly, that’s a bit of a muddle, they’d have to create a kind of new reality/take the cosmos into their own heads and edit it, first.