Not commenting on the last post, and I have no experience of that particular product.
I had a well recommended immobiliser on a Landy a while ago. It was an armoured cover to an additional electric switch to the fuel solenoid bolting to the injection pump, that without the coded electronic key shut the fuel off. The mounting bolts were recessed and then covered by hitting ball bearings into the recess. It wasn't coming off! It was apparently widely used on earthmoving plant that often gets left out in the sticks with noone around.
Initially worked well, was unobtrusive, the "key" was no bother.
Poor quality diesel ended up making the solenoid sticky, so I had to stall it to turn it off and it ended up stuck open, absolutely no use at all. In fact, to change the solenoid, or service the pump was then impossible, and removing it meant drilling it out from the injection pump, needing a new pump if I hadn't sold it first. Too much hassle!
Chain the steering wheel with a padlock you can cut if you lose the key!