Back when I installed cellphones for a living, alarm installers were the bottom of the barrel for electronics installers. Slap and dash was their style. Normally, it was horrible work.
I doubt it's much better today, but there are always good shops here and there.
I was an installer/upfitter, building cop cars, fire trucks, ambulances, etc. and yes, even did a stint installing car alarms and stereos.
When you had to tap VSS wires, run lines throughout the vehicle, make it idiot proof and **** near invisible, doing a stereo or an alarm is child's play. I used to take pride in asking my clients to find a non-factory wire.. I've even done some surveillance units in sedans that were wired out the @$$ with A/V equipment, radio, and even lights/sirens.. and even if you inspected the car, you wouldn't find anything unless you started tearing panels apart...
It's all on who you hire. I've worked with some amazing people, they are out there... BUT, I share your caution, because a lot of people don't know what the #(%& they're doing..
I've definitely had to clean up other shop's work.... and I've even had shops ADVERTISE their crappy work while claiming it was a mona lisa..
Anyway, not a guarantee, but look for a shop that has MECP techs. It's usually a good indicator.