I have never heard of the transport companies losing or having luggage stolen, although I did hear once of a pack going missing. The person spent the night without their “stuff,” which I am sure was inconvenient, but was reunited with their pack the next day. I do think the way luggage is left sitting around unattended seems unsafe, but maybe it just isn’t that tempting to thieves. And a lot of smaller places, private albergues and other lodging, do take care to keep it safe and out of sight in storage rooms. Some even will check the names on all luggage deliveries to make sure it belongs to one of their guests.
I do hope it goes without saying that anything valuable should always be with you.
One thing I think people should realize is that you do need to carry quite a bit with you even if you send your pack. Rain gear, sun protection, a warm jacket that you put on and take off, water, snacks, extra socks, a blister and first aid kit, and so on.
I used transport service last year due to a pinched nerve and various joint problems. However, I still carried my pack, but took out things I didn’t need while walking and put them in a small duffel bag that I had brought for that purpose. My husband did the same. Our packs were lighter, but still probably 8 pounds/3.5 kilos with water.
I put a lock on the duffel. I don’t buy that logic that it attracts thieves, who are opportunistic and want to work quickly. A lock will slow them down.
I always lock my luggage.
One last comment for folks with backs not strong enough to carry a pack … why not do something about that? Even a couple of months of simple core strengthening exercises can make a world of difference.