Yeah I remember seeing that set up, it's just been a while.
What we fail to realize sometimes is that on these engine kits the engines are started by "popping" the clutch, when this is done it throws all of the chain tension to the lower chain and if there is a spring tensioner there the sudden release of the clutch will overpower the spring and cause the chain to get very loose and many times that is when the problems start, chian comes off posible tangles up and locks up rear wheel or might even break spokes on wheel......at any rate NOT good..!
Now if the spring loaded tensioner had a possitive stop that would not allow the tensioner arm to drop to an extreme point when the spring was over powered then it would likely work just fine that way.
The problem is when the engine is started or when backing off the gas while the bike is going down hill, that is when all the tension is moved to the tensioner rather than being at teh top portion of the chain like it is when you're under power from the engine.
Here is a pick of what I put together for one of my bikes and it has never given me a single issue in 550+Miles so far.
Hope this helps and possible answers a question or two.