Most genealogists have a love-hate relationship with online family trees. Many have mistakes, some noticeable. If you've ever seen a tree with an impossible fact - such as being enumerated in a census after death - you'll know what I mean. Others don't list their sources. That can make strange facts hard to confirm. I have one ancestor whose birth date is chronically listed off by one day, simply because someone misread his original birth certificate. If I hadn't backtraced the story, I never would have figured out what happened. But there are some good trees out there, especially for Luxembourg genealogy.
I've mentioned Bob Arens's Luxembourg American Families before and wanted to mention it again. Arens has made an effort to trace many of the Luxembourg-American families and provides good basic information on many families. If you find something wrong, he's provided a way to submit corrections.
Why am I mentioning his trees? Because right now, it's providing motivation for my research. He's got more information than I do, but I've found errors in the information previously submitted to his tree. End result, I'm doing what I can to confirm what he's found. And hopefully, we're both benefiting.