My opinion comes from many years of observation. I have had many tai chi practitioners come through my classes; the oldest had practiced tai chi for 90 years! What I found was a completely different understanding of energetics and that the energetics do differ significantly from qigong; and let me be clear I am really speaking of neigong versus what some call qigong that are more calisthenic oriented or are simple breathing exercises.
I have found Tai Chi to be a great physical exercise that also produces many mental as well as physical benefits, also, in the classical TCM sense, helps restore energy circulation through the channels. I have no problem with this and do suggest Tai Chi as a great exercise. But when it comes to transmutation and raising the vibration of the energy body - not so much.
There was an interesting article about the history of tai chi in one of the MA magazines a few years ago. And that was about the reason it is currently taught to do slowly. Seems that (a long time ago and far away) one of the top teachers injured himself and during his recovery had to do the movements slowly because he couldn't do them fast like what was currently taught. So everyone else started doing it slowly due to the current top master doing so. Of course I realize the popular view among the martial folks is that it is done slowly so as to learn the exercises (which I agree with) and the popular view among the tai chi-as-qigong folks it is done slowly to maintain proper alignment for energy circulation and to feel the qi.
One of the differences of opinion about Tai Chi is due to the fact of the difference in the way people are taught. I was shown each move's martial application which adequately demonstrated Tai Chi is a vicious and brutal martial art. Can a person slow it down and utilize it as a "qigong"? I say yes but with certain caveats. One is that a qigong form (preferable an internal form) should be practiced concurrently with a person utilizing the qigong energetics inside the tai chi. The other is that there are certain carryover energetics from the fact that each of these moves were designed to maim someone (stop the heart, break the jaw, displace the shoulder etc). An attached "karma" or perhaps just to say "attached energetics" would be one way to look at it. I can demonstrate this with a field of horses. If I do Gift of Tao neigong movements the horses are attracted and come very close and get in a really relaxed droopy jawed, almost closed eye state. The very second I switch over to either tai chi or hsing I (with no change in body tension or sudden move) the horses jump to attention and the leader walks out to challenge, although he does have a confused stance as if to say "But you are my friend, why are you challenging me?" It is quite obvious to me that they are picking up on the energetics difference. And, I have performed this experiment many times and with different horses.
Also, I have met very few who could adequately perform Wai Qi Liao Fa from doing only tai chi.
My own personal conclusion is to practice neigong alongside the tai chi and integrate these energetics inside the movements. Although from my experiments with animals (PETA, no animals were harmed during this testing) I also conclude there is no way to separate the martial energetic attachment. Dilute it, yes; separate it, no.