Hmm, I think you're right to be a little wary of such an offering, unless of course, this is an eight year program.
In Canada (and I believe it's similar in the US), a law degree is a professional program and normally admission into the program is so competitive that you need to have completed a prior degree as a part of the admission requirements (or if not, a couple years of university along with a set of prerequisite courses). So the first flag/question that comes up is how competitive is this joint degree program to get into? Is it on par with other law programs in your country? Engineering on the other hand tends to admit students right out of high school.
Secondly, while there is some overlap of the fields - patent law, ethics and technology, etc., the study of each field will largely be independent from the other with little if any overlap of course material. This presents you with the dilemma of spreading yourself too thin in either field - you may cover the basics, but those who studied only one of the fields will likely be exposed to a lot more within it. This is a common concern with any double major, but when the two majors are related like, say physics and mathematics, or physics and computer science, you can benefit from a synergistic effect. I don't mean to say it's impossible to combine the two. There are examples of successful lawyers who started with degrees in engineering. But I would look long and hard at how the university approaches this concern.
Building on this point there's also the question of the education to professional transition. Because these are two distinct professions and the niche areas of overlap are relatively small, there's a good chance you'll end up working in one or the other. Are you going to be upset if you end up working as an engineer, and not have any professional opportunities to put your law degree to work? If the join program is that much longer, you might also want to think about the opportunity cost. If you did straight engineering you would start working (potentially) four years earlier. That's fours years of paying down a mortgage, four years of investment opportunity, etc.
Finally, there are the common concerns about any new program. When it comes to something like your education it's nice to judge a program by where it's recent graduates are ending up. If you see that the program's graduates are ending up as lawyers, do well on their professional exams, or are ending up as sought-after engineers, than you have reasonable grounds to expect to end up in a similar situation. But as part of the first couple classes you might be a guinea pig.