Is building a spec home less risky than flipping a fixer upper?

[Patrick Britton]

I have been looking at spec building since the early summer and cannot seem to find the same level or amount of risk associated as with fix and flips.  It seems that because nearly all of the costs are known well in advance, the biggest risk is whether the home will sell quickly and for the desired amount.  But this is the same risk one takes with a fix and flip...

Considering the risk someone takes with a fixer-upper with respect to unknown repair needs, would it not be a better (less risky) strategy to do spec builds instead?  After all, the construction costs are known well in advance for the most part.  Seems like the most important factor is building a home that will sell for the desired price quickly.

Are there unknown or considerable risks associated with spec building I am not considering?

Please assume that the builder is reputable and can finish the job in a reasonable amount of time.  thanks