a conversion is as safe as the competence of the mechanic building it. if you are comfortable with your mechanical abilities and have a very thorough work process you'll be just fine. but try and breeze through it for pennies on the dollar and you may end up knee deep down **** creek without a paddle.

on the other handle you could be very methodical and have a freak failure while the penny pincher on a scrap yard bike rides on for years. it is impossible to predict the safety of used components. however, better judgement can be used to build a cycle with the best probability of functioning as designed.

something else to consider. i built a fixie conversion last week and it snowed today. it may not be unsafe to build one, but it will screw with the weather. science proves it.