With very few exceptions, they mangle the pronunciation of every German word. For example, in the word "Wesen", the first "e" is actually long and the "s" is soft.
The crossbow is supposed to be hundreds of years old but it has a modern cocking mechanism.
The trailer is more than twice as wide on the inside than on the outside, revealing the interiors were filmed on a sound stage.
The lid should have been blown off the Wesen/Grimm secret a long time ago by normal humans adopting orphaned Wesen children who at some point started to woge, with no family tradition or parental guidance to explain it all.
Ever since transplant surgery has been established, there should have been lots of incidents of surgeons inadvertently transplanting Wesen body parts leading to inexplicable medical complications and blowing the lid of the Grimm conspiracy.
The logical thing to from the start would have been to enter all the creature data from the ancient books into a digital database to convert it into accessible, usable information that could be searched and cross-referenced.
At no point does Nick try to find out if there is any forensic way to determine from bodily evidence if a person was a Wesen after they died. If there were,that would sure have come in handy in the many cases when Nick only learned that information from context or circumstantial evidence or witnesses some time after the body was discovered. Nick should at least have started asking around about that at some point.