It still has some flaws in my opinion. For example, sometimes runtime errors happen when you deploy it while there are no errors when you run it locally. Unit/Widget tests don't work with dart:html library included, or the already mentioned slow startup time. There are also some packages that are not supported for web, so you have to be careful with those. However, I have been using it for over 2 years now and overall it's a great tool to work with!