I was on the last day of a tour before leaving Morocco about five years ago. One of the guys on the tour had a bike with an after market alarm/immobiliser fitted. It absolutely pissed down, I'd never seen rain like it before, waterproof clothing was useless. He stopped in a garage to fill up with petrol and the alarm/immobiliser refused to disarm when he tried to start the bike again, leaving the guy stranded. It took the combined effort of several of us to get the bike going again, which delayed us the rest of the day and a VERY late arrival at our hotel that night.
Roll back even further, about 15 years ago, I was in Bournemouth working in a small secure building with no windows. I'd parked up outside and noticed three bikes parked in the small car park. I'd been there about half hour or so and I could hear one of the bike alarms go off. I went to the door and looked outside to see four scrotes lifting the bikes into the back of a scummy old van. I shouted at them and they dropped the bike and legged it. Other folks were walking by taking no notice at all.
Personally, I think alarm/immobilisers on bikes are useless and can be a right pain. So more trouble than they are worth, get a decent lock and chain.