I've not intended to suggest the two cannot be done simultaneously, and they certainly can for novice and intermediate level lifters. My real concern is joint laxity, but as Dr. El Hewie mentions, muscle flexibility and joint laxity are not necessarily connected. That said, my concern specific to yoga is that in my experience, yoga instructors and practitioners have no experience with strength training and accordingly have no genuine appreciation or understanding of its demands and unique characteristics, and many pursue flexibility indiscrimately, causing irreversible damage to inelastic connective tissue, as well as flexibility well beyond what is appropriate.
In regard to flexibility itself, there is certainly such a thing as excessive. Flexibility is entirely discipline-specific. What's appropriate in terms of degree, nature and location for one discipline is not for another. Weightlifting is often cited as demanding remarkable flexibility, but this is only partially true--it demands a great deal of flexibility in very specific locations and manners. Flexibility beyond the necessary ROM plus a small margin of error is likely not much less risky in terms of injury potential as inadequate flexibility.
So the bottom line is that you just need to assess your priorities and determine what flexibility will best serve you. You may decide that flexibilit beyond what is appropriate for weightlifting is worth it for your pursuit of yoga--Just recognize that it's a compromise.