Rendered beef fat as a moisturiser has gone from fringe to everywhere. It is worth taking seriously enough to look at properly, because the arguments on both sides are mostly vibes.
The case for it
Tallow is largely oleic, palmitic and stearic acid, with some vitamins A, D, E and K carried along. Those fatty acids do appear in human sebum, and the claim that follows is that tallow is therefore closer to what skin makes than a plant oil is.
There is something to that. It is an effective occlusive and emollient, it is cheap, and people with very dry skin often report liking it. As a barrier layer on dry skin, it works, in the way most heavy fats work.
The case against it
Oleic acid is the problem. It is high in tallow, and oleic acid is fairly well documented as disruptive to the skin barrier at concentration, and comedogenic for a lot of people. It is the same reason coconut oil suits some faces and breaks others out badly.
If your skin is acne prone, this is not a neutral experiment.
What the research actually contains
Almost nothing. There are no meaningful controlled trials of tallow as a facial moisturiser. The mechanistic argument about fatty acid similarity is plausible and untested at the level that would let anyone recommend it confidently.
That cuts both ways. The people saying it is obviously wonderful and the people saying it is obviously terrible are both working from theory.
The honest summary
If you have very dry, non-acne-prone skin, tallow is a reasonable occlusive and you are unlikely to come to harm trying it. If you are acne prone, the oleic acid content is a genuine reason for caution, and there are formulations that do the same job without that risk.
What it is not is a replacement for a routine. It is one layer, doing one thing, and doing it about as well as several cheaper and better-studied options.
The thing worth noticing
The appeal here is mostly about trust rather than efficacy. Tallow is a single recognisable ingredient at a time when ingredient lists feel opaque. That is a real and reasonable feeling, and it is worth separating from the question of whether the ingredient outperforms the alternatives, which so far nobody has shown.