Does fusible tape really work, and are there any limitations to its use - such as limited durability or restrictions for washing and drying?

It can really work, but it is tricky to get right. Different pants' materials, their weave, and their weight all can affect how the tape attaches, and then there's the challenge of heating up the hem and tape enough so that it melts completely but you don't burn your pants. And even if you get a good melt and bond, it can look just exactly like you glued your hems in place. I have been working with fusible tape of various types for over 30 years and I do not use it to hem pants of any kind because it just doesn't turn out that well 90% of the time. It is so much easier to use a needle and thread and the result is far better.