One of the reasons I left freighting.
Consider this:
1. Usually (but not always) old aircraft who've finished their front-line passenger service. This obviously does not include purpose-built freighters.
2. Usually operated at the maximum possible ZFW (unless bulked out) sporting 'tired' engines in many cases.
3. My experience was operating into Africa (particularly) often at night with minimum aids.
In a nutshell, old aircraft working harder than they ever have, often with less than ideal serviceability, and yes, often operated by crews that can't get a job elsewhere (not all by any means, but more than my experience of the average ability crews elsewhere - doubtless I'll take flak for that, but I can only say it as I saw it).
So, yes, it can be hazardous. That's why being with a good freighting company cuts down the risk.