About the ME algorithm used in x264

The UMH algorithm has been proposed many years. Why x264 always uses this method? Because these successors performance worse?

x264 uses HEX by default, not UMH. UMH is used as a slower search for the slower encoding presets. It uses this method because someone wrote code for it. It doesn't use something else because nobody has written a patch to implement something else.

ME algorithms are not exactly cake walks, both from a implementation and understanding standpoint. It's one reason why GPU accelerated ME still has yet to produce any real results. It's not easy to just whip up an ME algorithm, or C implementation of that algorithm. As Dark Shikari said, if you want a new algorithm introduced into x264, write a patch and submit it to the devs.