To the uninitiated - which is what this guy looks like - Concorde can look dodgy. However it is a different type of plane so its ability to retain control and its sensitivity to stall allow narrower margins whilst being as safe as an ordinary jet.
All the evidence I've ever seen shows certification to have been as rigorous as for anything else.
Concorde would have survived on two engines I think. The problem was loss of control which was not caused by lack of power but rather lack of physical parts of the wing.