Smoke Alarms yes simple but like anything electrical comes with a Caveat. 

1/ I find the maintenance cost high, but note if your house burns down then the onus is on you to prove that the smoke alarm was working???

2/ Just by pushing the test button does not mean the alarm is working, the only real way is to use synthetic smoke, or put your toaster underneath and burn some toast.

3/  Also remember that  whilst you pay insurance premiums, you cannot be sure you are actually insured until the insurer pays up (that is the test).

4/ If you use a third party and your house burns down and you can prove it was because of a faulty or poorly maintained smoke detector / alarm you may be able to sue the maintainer if they have professional indemnity insurance, always worth asking prior to engaging any person who does work on your property.

5/ Builders often use third parties for difficult or high risk work such as membranes in wet areas, the only reason is if it fails they have recourse.