I had 24-105 and sold it for the tamron..my copy is perfectly fine and it also has a 6year warranty so don't be affraid.
Buying from third party companies like Tamron, theres all the time risk that something will be wrong. I have canon 550D and Tamron 17-50mm f2.8 VC for about three years. I'm not happy with 17-50mm f2.8 VC, difficult to get it sharp, its soft at f2.8, sometimes I get errors, time to time a lot of mist focus shots and sometimes I even not sure if it suppose to work like this. You must to get used to them and when you'll be able to make good shots.
Took only once this lens to beach and second day discovered pieces of sand inside. But I was also not happy with canon 17-55mm f2.8. It was very sharp, but build quality was very weak, maybe even worse than Tamron. I send canon back after one week.
Thats why this is one of my reasons to move to full frame. Still not sure if that will be 6D or Mark III. Im not professional, but one or another decision have its own pluses and cons.
The same dilema about lens. I can try to risk again to move to tamron 24-70mm, but who knows how many weeks I will lose if something will be wrong, replacing it over and over. I have tamron 70-300mm VC and they really improved in focus system. At least I know if I'll go with Canon 24-105mm f4 theres much less risk and I'm not afraid to use it any time in beach or in bad weather condition. And If I'll have really low lite condition, I can use my Sigma 50mm f1.4 prime.