My brother has locked up his engine 3x in the last 6 months. One time ended with him on the ground with chunks of flesh missing from his leg and arm. He'd been riding motorized bikes for 2 years prior... always with the stock tensioner... then I bought him a spring tensioner.
Yup... all 3x he locked up he was using the spring tensioner.
No thank you.
I did say... assuming the proper spring is used and set up properly.
Are you referring to the chain skipping a tooth on the drive sprocket on the engine and jamming in the case there? If so, there was likely way too much slack on the chain, and the spring idler couldn't do its job maintaining pressure and essentially wasnt working. If the spring tensioner is set up properly with proper spring tension (hence why most have multiple spring positions), it will not move much while under load. Maybe the width of the chain worth of movement. If its swinging more than that, your chain is too long and/or spring pressure too low. You cant just slap them on there and expect them to work properly. They require adjustment.
There is some variance in the springs that come with those tensioner kits. Maybe you got a wimpy one that was causing the troubles.
As chainlube suggested with his unit, it is indeed mostly about taking up the slack rather than applying a specific tension, a spring tensioner does that dynamically according to load.
Another benefit of the spring tensioners is they automatically take up the chain stretch and dont require periodic adjustment if you are using a crummy stock/kit chain. Which is another subject entirely.....
I have never had a chain jump with a spring tensioner, but I have with a rigid tensioner. I have personally had better luck running no tensioner over a rigid idler. But that requires careful alignment.