Fans have every reason to be alarmed. Not only have video game to movie and TV adaptations been almost universally bad, but Mass Effect is in my opinion, a relatively bad property to adapt. Costs aside, it either has to go the prequel route or run parallel to the games meaning it has a very small window of time before the Reaper invasion. Which they certainly could show, again with cost being a problem to do the scope and scale of that well.
But beyond that, everyone's vision of how that story goes is different based on their playthroughs. It can be done, and something similar was vaguely done with the Witcher TV show (up to a point), but there is a big difference. The Witcher is an adaptation of novels first before it was a game series. So there is a single canonical way for that to go. With Mass Effect, it was a game first before it was anything else. So the right path, gender and look for Shepard is different for everyone. They could do this with someone other than Shepard, which is probably the safest way to do it.
Beyond that, without getting too political about it, the concern is always that the adaptation will be for "Modern Audiences" which never goes well and is the death knell for any TV show or film adaptation. My guess is this will go over about as well as the Halo TV show did.
