I've taken Eurostar from London and changed in Paris for Grenoble and Brussels for Amsterdam (before the direct train.) It was completely fine. No different from any other train journey with a change.

I think it may have taken a while to get from Brussels to Amsterdam because that line hadn't yet been upgraded at the time but the journey time wasn't far off what it would have been getting to and flying from Heathrow/Gatwick/Stansted.