With Earth Shake into dash with them crossing up, you're at free reign to do whatever you want to them. Like, anything. No meter burning required. If Body Splash doesn't actually hit them, it still leaves them in blockstun for you. If they block MB Up Venom, they punish you.
It costs too many resources for an unsafe, MB mixup that can replaced with a cheaper, safer option, effective or not, which I don't think it really is. If someone has seen this mixup, all they need to know is to duck. Ordinarily this'd be hard to see, except for the very obvious telegraphing that is Upward Venom. It takes 24 frames for people to realize that all that is needed is to hold down, you're out meter, and they get a free full combo.
So to recount:
-Splash is easier to see, but safer, cheaper, and leaves them in blockstun
-MB Upward Venom is more ambiguous, but doesn't crossup, costs meter, telegraphs hard, and is vulnerable.