Cancer detected in both control and suplplemented group
That's not a flaw. I think researchers are trying to create cancer in these tests, so that's part of the design. You use mice that are prone to cancer. Then what you want to observe is whether your test substance, NR in this case, increases the incidence of cancer. Which it did.
They used mice specifically bred to not have a thymus or T cells
I don't know much about mouse models for testing suspected carcinogens, but it may be that this T-cell knockout mouse model is often used for such testing. But I agree that if the main anti-cancer effect of NAD+ is medicated by T-cells, then it may have created an artificial situation by using T-cell knockout mice. So this could be a flaw, admittedly.
The NR dose given (2,500 mg/day human equivalent) is far higher than the oral NR dose typically used by people
This is not necessarily a flaw. A high dose given for a short time may be equivalent to a lower dose given for a longer time. Many people taking NR for its assumed longevity effects are taking it daily for years on end.
This will not be the first time that supplements touted to increase longevity have come under scrutiny: decades ago, it was posited that daily antioxidants like vitamin C would increased longevity. But dozens of studies have proven the opposite, that vitamin C supplementation does not increase longevity, and is actually associated with a shortened lifespan.
More recently, longevity enthusiasts started taking C60, only later to discover that this caused cancer in mice, under certain circumstances (when the C60 is exposed to light it becomes carcinogenic).
The title is misleading:
Nicotinamide riboside (NR), a form of vitamin B3, increases risk of breast and brain cancer
It looks like Science Daily have now updated their title from what was originally:
Popular dietary supplement causes cancer risk, brain metastasis
Popular dietary supplement increases breast cancer risk, brain metastasis, study suggests [Updated]
So I've just added "study suggests" to the title of this thread.