The Holsteiners I have seen, admittedly not a lot, have varied quite a bit and so not sure I could say anything about them as a group and their suitability for eventing.
My last event horse was an approved Holsteiner with a decent dose of TB (dam was TB and there was TB blood in her sire as well). She had a nice combination of a great gallop and nice movement. She also had some qualities that were not great for an event horse. Not sure if you could say any of that was because she was a Holsteiner. Her build was more TB and not heavy WB at all. I don't think Holsteiners tend to be the heavier kind of WB (which I would personally not like although they can be successful event horses, at least at prelim and below- not sure how many can do upper levels)
There is something about the outside of a horse that is good for the inside of a man.(Churchill)
Cavalier Royale was a Holsteiner who sired many upper level horses. They tend to be good jumpers, and you'll see them in the jumper ring more than the dressage ring. So the short answer is yes, they can make good eventers, but like any breed you need to know the horse.