Sure, you can use flex watt.

The reason why most people don't is because, usually, those who use tank set-ups have a very small collection - usually 1 - and so buying flex watt and having to assemble it yourself is a pain for the measley amount of savings you're gonna get. Flex watt is not readily available in stores. Usually, you'll have to have it shipped to you which is $9 for standard shipping. The flex watt itself is cheap - about $2-$3 a foot but then, most people get the connector sets and insulators which would then make the foot of flex watt inch up to almost $20.

You can buy a zoomed heat mat for not much more than that.

The danger to flex watt is the same for the heat mats - that is, if you wired the flex watt properly.