I personally would recommend a tube whether you do the mercury bulb or not. I know people have used them successfully and they definitely provide more UVB than the coils, but some of the other limitations are similar in my opinion. Tube gives more coverage and more options for the dragon. With a tube you essentially have four light/heat zones that the dragon can choose from. Basking spot for high heat and high UV exposure, floor of the warm side for some heat and lower UV, another spot on the cool side that's closer to the UV so you have low heat and high UV exposure, and then the floor of the cool side or preferably a hide where they can go to get away from both. With the MVB, the UV is also tied to the heat, and if the basking spot is getting too hot and you needed to raise the lamp or dim it, you're also lowering the UV. So, long story short, it is my opinion that MVB is great if it is used mainly as the heat source and not the sole UVB source, and they are also really well suited for a basking spot outside the enclosure if you let your beardie free roam a lot.