stuart, a good rule of thumb is no more than 2 90 degree bends for a regular style wood heater, a wood stove (I sometimes call interchange the words too) should really have the best draft possible so trying to limit it to only one 90 degree bend in the flue pipe is a thing to strive for. These are by no means hard and fast rules and as long as your stove will draft, you can tie the flue in a bow knot haha.
the issues I can see (aside from slowing down the draft) is making the flue harder to clean and that you would need to support the flue pipe if it had a horizontal run supporting a vertical run.
when you slow down your draft too much in a conventional wood heater, the smoke can cool before it reaches the end and that will cause quicker buildup of creosote in the flue and a heater/stove that will smoke the house up from time to time.
hope that helps, and no I can't quote my sources haha, just a lifetime of heating with wood and being in one too many homes with a poorly plumbed heater
