The downside is that the disc brake bike won't get the advantages of a disc-specific rim, like greater structural efficiency (how strong it is for its weight) and ding resistance due to no brake track, and the rim brake bike won't get the advantage of a dishless wheel, which is greater total tension and more strength.
Building such a wheel does mean the worst of both worlds. That said, front wheels often have pretty easy lives so it's not all that likely to make any kind of difference in use unless you're pushing the limits of the wheel. If you do want to steer towards getting all the strength and durability you can for the weight, enough is lost here that I wouldn't do it.