Both dramas contain the element of time travel into a parallel timeline, where the MC attempts to change the past.

Both are wuxia with mystery and awesome action. Both have good CGI. ML rise from young age to become powerful cultivators, starting from being extremely weak. Cultivation school life also covered a bit like BTTH, ML is a prodigy in school. Both have powerful and kind masters. Good master-student relationship. Both ML are righteous and upholds the ideals, they both have precious secrets about them. Both have a big revenge to take . Strong and beautiful female leads too.
Sword dynasty also has good romance. In all, it is nice to watch.

Both dramas have the wuxia element with mystery and awesome action. Both have good CGI. Both are adapted from the novels of the same writer. MC rising from grassroot, both were considered trash but through miracle and their tiresome cultivation, they became most powerful cultivators in their respective universes. Both dramas consist of strong female leads. Both fought hard to uphold ideals and be righteous, battling the evil. Both had powerful masters to aid and guide them in difficulties.
Both pay some amount of significance to inheritance of past old men and fight techniques, which have names unlike many other wuxia dramas.