Why does Migration Assistant get stuck?

I've tried transfers from MacBook Pro (2021) to MacBook Air (2025) and nothing works. Time Machine failed (supposedly not enough space, which doesn't seem right.) Wi-Fi was too slow. USB-C took nine hours and then got stuck on a single file. Thunderbolt took twenty minutes before it too got stuck — a little further along, but still near the end — and then stayed there for over an hour. (It's odd to me that iPhone transfers are so seamless by comparison, but I digress.) Any idea what might be wrong? Thanks.