I note they also say they have a linguist and a Chinese culture expert ready to testify if necessary. I am assuming they are anticipating that BC is going to try and claim that this was a consensual act and that she was accidentally killed. Or they may try to claim she expressed interest in a kidnapping fantasy online or in her diary, and they may try to claim that she was embarrassed by what she did with him and that she fled and is in hiding because of shame. I don't think that any of that will fly, and that the two experts they mention will be called if they try to go there. I note that her diary is in evidence, so it is possible she could have written some things in it about her sex life, fantasies, whatever. However, I really doubt there will be anything in there that provides much help to his defense. The FBI allowed the University to make photocopies of every page of the diary, which were then bound and presented to her parents, and the diary was then returned to the FBI. If there were anything sensational in there that undermined their case or would need more investigation to be explained, I seriously doubt they would have allowed copies to be given to her parents -when at the same time they haven't shared many details of what they know about what happened to Yingying or about the search for her.
I think it's natural that everyone worries that their case is weak and that he might get away with it. While we must always remember that he is entitled to the presumption of innocence, it seems pretty solid and clear that she's dead and that he did it, even though we have not heard his side of the story. However, because investigators have been so tight-lipped about what they have, and what they know, it's natural for doubt to creep in. So, let me remind everyone of something:
Back when she disappeared and they released video of her getting in to the Saturn, the search was on for that car; and it went on. And on. And on. And on. For weeks. People started pulling their hair out online. "Where is that car?!?!" "How can it be taking this long to find that car?!?" "Why haven't they found that car?!?" "They only made ~20,000 of those things, how can they not find it?!? Damn, what a bunch of inept bunglers!!!!!" No one had any clue what they had. Meanwhile , the very day they released the video, they found BC, interviewed him and searched the car. Two days later, they concluded it was his car. The next day, they towed it away, searched it, interviewed him and put him under surveillance. The entire time everyone online was wondering where the car was and was out trying to look for it, they not only had found it, they were watching the suspect's every move 24-7.
They only tell us what we need to know that can also help them with an investigation, and right now, there's nothing we need to know. That doesn't mean they have nothing, or what they have is weak.
I think they have a lot. They were following his every move physically and electronically for two weeks. Hell, they probably put a tracking device on his Saturn and were detailing its every move after they returned it to him. After they arrested him, they had access to EVERYTHING in his apartment and were able to dig even deeper into what he had been doing. Oh, I'm sure he cleaned, and cleaned well -but he didn't clean thing(s) he used well enough.
Are there some holes that need to be filled? Probably. Not having the body doesn't help. I think they can probably get a conviction, but based on another case I've seen here, they might have trouble getting death without her body. We'll see though, as again, we don't know everything they have