A bit of reason is called for
I entirely agree that I would not use anyone else's loads....
Having said that generically, there is a certain degree of human reason with all things, and there are established cautions. If that comment were taken to its absolute conclusion, my wife would never shoot my gun or hers with ammo that I've loaded for her. Nor would my best friend, whom I've known for 55 years, ever trust me with loads that I help him work up. Likewise, that same degree of absolute logic would make every shooter at a benchrest match expect that the person sitting beside him or her would be immolated in a fireball at the next shot, with fragments of rifle barrels flying down the benches. As in all things, our God given human reason provides us with certain facts that we can relate to give us guidance.
Therefore, the more specific question is reasonably, where did this ammo come from, or who made it? If I know from personal knowledge and awareness of all facts that the person from whom I have received handloads to shoot uses the same criteria of caution and wisdom that I employ, I am quite happy to fire his gun or his loads, as they are to shoot mine.
Handloading is not a quasi-legitimate activity, nor is it voodoo, and it should not be thought dangerous when performed by competent, skilled, knowledgeable, and sober persons, in consultation with published data. In fact, handloads were around before commercial ammunition. In the mid 1800s, wives used to roll their husbands' black powder Minie ball muzzle loads and put them in his pouch to march off to battle. Most black powder cartridge repeaters saw more loads made with old Ideal "nutcrackers" at a kitchen table than they saw from Remington-Peters cartons.
We have established mountains of reliable data on which to rely, and we have every reason to believe that other handloaders that we encounter in our lives are trustworthy to perform a simple job of making a cartridge with utter and complete safety, and that person in return. Likewise, and it should go without saying, that if someone is a fool in other areas of life, it behooves us to not get close to whatever that person does, whether it has to do with loading ammo or frying turkeys for Thanksgiving.
If one cannot declare with absolute certainty that their own loads are safe to put in their own gun or someone else's (custom fit is entirely a different issue), that person should seek experienced help and guidance, because nothing about handloading is mysterious or subject to luck. It is a very perfected science that has very defined parameters that are spelled out by all loading manuals and powder company literature. Nobody with less confidence than this, should ever engage in handloading. I see many individuals on this and other sites who appear to approach loading data as if it were a course in handloading. It is not, any more than a list of ingredients constitutes a course in baking bread. Likewise, I see folks who are entirely new to the game talking about setting up a Dillon to load thousands of rounds, asking blithely, "How much 2400 can I use in my .44 Special?" This guy doesn't even know what 2400 is from popping corn. That, my friends, is the fool you want nowhere near your shooting position. It will never occur to him that handloading must be learned and understood after reading at LEAST one entire authoritative book on the subject--with comprehension--before it is done. If one cannot justify that time spent, indeed, he should shoot far off, by himself, with his life insurance up to date.
I've been enjoying handloading for over 40 years, and have taught many folks how to have that same enjoyment, with complete safety. The simple fact is that when a dear late friend of mine made me boxes of .222 Remingtons in the early 60s so I could go chuck shooting with my Sako Vixen--and I can recite the load he wrote on the Sierra label from memory--I shot them with complete confidence, and the accuracy of those loads inspired me to pick up the game myself, and I never looked back. I knew that he was an intelligent man, a local bank president, who had many benchrest and varmint rifles that he competed with using his handloads. I had infinitely more confidence shooting his ammo than I had eating at the local burger joint! Moreover, I loaded hundreds of thousands of .38 specials from my department's Star Universal loading press, and hundreds of our officers shot them. Taken to an absolute conclusion, the admonition of never shooting anyone else's loads would have given them reason to sit out their qualifications for fear that guns would be blowing up in their fists. But, they had confidence that my partner and I shared a labor that we took seriously for their collected safety. Another extension of such an absolute mindset would dictate that one could never confer with anyone else about a load. Of course one may.
I might ask my best friend, "Pete, what's your pet load for the .270 in your 700?" When he tells me off the top of his head, "A variation of the old Jack O'Connor load of H-4831 with a Sierra 130 grain boat-tail. My gun shoots 59.5" I should not be surprised when I double check printed load data that his load is indeed within the parameters of articulable data, oft printed. There is a caveat to that. Because of our close association as friends over the years, and because we have foreknowledge that I would not just stuff cases full of said load without working up my own load for my own rifle, incrementally, we can communicate freely, understanding that the rules of the game are never suspended. I can assure you that the people I know personally are far more careful and particular--to a very fastidious degree--with their handloads than half the people behind the wheel of the cars I see coming at me in the middle of the night.
So, while it is foolhardy to use anyone else's loads", one might confidently and safely use someone else's loads.
Viva la difference!