I think this issue has been well covered , but there are differences of opinion.
The cap is a band aid.
If you brace the housing and add the cap , there isn't much more to do.

Making sure it is well set up and using quality parts goes without saying.
With a stick and a lot of weight ( 3000 or more ) I don't like their odds.
With an auto , a light car and foot braking , it may last forever.
I believe that weight is a big issue for them , but a stick and the shock they impose on the driveline just compounds that.
If it hooks hard , I really don't like their odds.
I am in the 9" camp , but the Dana is also a decent choice.