Assembly is easy - the only tricky part is the brake assembly, which requires some effort to get it right. All other parts are just put on, screw tight, hope it holds. All parts fit and assembly (with brake) was done in half an hour.
Overall quality is my major gripe here - the pedals are not spinning easily, but are stuck time and time again, the clamp that arrests the saddle does not have my confidence, the training wheels attachment is flimsy, and so on.
I do not trust this bike to last more than half a year, something's gonna give before that. Probably one of those tiny parts that are essential to the bike (like the pedals, saddle clamp, etc.) and can not be replaced easily.
I do not want to state that the bike is unsafe or even hazardous - but in my opinion, it is not built to last and the quality in some areas is quite poor.
Would I buy this again? Sure - my little princess loves it, so far nothing broke off, and who knows, maybe it lasts until she needs a bigger bike. At this age (she is four ...), color and tassels are much more important than good functionality - and I do not see any big hazards that could harm her.