Mixamo is genuinely good and genuinely free, and it is worth being straight about that before arguing with it. Upload a humanoid, place five markers, and it hands back a clean rig and a library of hand-cleaned, loop-ready animations that you can download as FBX at no cost, for commercial work, with no royalty. If all you need is a rigged character with a walk cycle on it, that is a very short path and this page is not going to pretend it isn't.
Two things about it have aged, though. It is a service rather than a tool, so everything it does happens on Adobe's servers behind an Adobe ID, and your character has to go up there to come back rigged. And it is a dead end by design: the FBX it returns is the last thing it does for you. Retiming a clip, blending two takes, fixing a joint that landed wrong, or doing anything to the character other than animating it, all happen somewhere else.
Mixos inverts both. The auto-rigger is code running on your own GPU — no upload, no account — and it measures your mesh rather than fitting it to a stock proportion: joint heights, arm span, shoulder width, crotch and ankles are read off the geometry, and the joints a silhouette cannot show are placed by human proportion and then snapped onto each limb's medial line. Because it is a tool and not a service, every joint it places is an ordinary scene object you can nudge before re-solving the weights.
The honest trade is the library. Mixamo's clips are curated and cleaned for game use; the 2,546 takes here come from the CMU motion-capture archive, which is research data — broader and stranger, with far more of it, but variable in quality and not authored to loop. You get a preview of each clip running on your own character before you commit it, which is the practical answer to that, but the two libraries are not the same kind of thing and picking a clip here takes more judgement.
The other honest trade is price, and it runs the other way from most of this page. Mixamo is free through the download. Here, rigging, retargeting, previewing, keyframing and rendering to the viewport are free on any account with no watermark and no trial clock — but taking the finished animation out as a GLB is on a paid plan. If a free FBX of a rigged character is the whole job, Mixamo does that and Mixos does not.