We have used Metacam injections a lot over 20 or so years. We have Zwartbles and our girls tend to be on the heavy side and have had various age related arthritis issues.
We've been down the injections every other day route. We've tried dissolving aspirin and squirting down the throat. We've tried homeopathic treatments and laser. Sadly, nothing proved the wonder cure we hoped for, but some things definitely helped.
As has been said, injections aren't nice for the sheep and are rather a trial for us too, so we have finally gone down the oral Metacam route. In fact, what we use is actually called Inflacam. You draw the liquid into a dosing syringe and put onto food, or squirt down the throat (which is what we do). Our sheep came running for their dose.
Having said that, yes - it isn't good for their kidneys in the long term so there's a definite case for careful thought.
And also, it isn't a miracle treatment and there comes point when you just know that the pain is no longer being managed well enough.
Hope this helps.