The remote starters are the ones you have to worry about in terms of affecting vehicle operation. The only critical wire that the alarm taps into would be the starter, but I haven't bothered with the starter interlock on my installs because I don't want to create the possibility of the alarm disabling the car against me if it's glitchy. Any other alarm problems would create only a minor inconvenience as long as you know where to find the fuse. I just want the car to make some noise if it's subjected to impact or intrusion. The first three installs I did were primarily done for keyless entry on older vehicles; the alarm was a secondary benefit.
I've installed four aftermarket alarms and had no problems with any. One was a Python, three were Autopage. They have had a couple glitches over the years that were solved by pulling the fuse, waiting a few seconds, then re-installing. All the wire-to-wire connections are soldered on my installs. I wouldn't want to pay someone to hack into my wiring with scotch locks.
I have a factory alarm with shock sensor and an immobilizer on my Mazda3, so that's good enough for me. The shock sensor wasn't OE; it was installed for free by Mazda as a TSB after the early models were found to be prone to break-ins where the alarm was disabled and the doors would unlock by denting the door above the driver's side handle. They also installed a plate in the door to prevent that issue, through another TSB. I also snipped a wire in the door so that the alarm cannot be mechanically disabled using the key mechanism. My only complaints are that it has no warn-away for light impacts, and I can't arm it while the vehicle is running. So I simply don't leave the car idling without me.
I think it's even more important to have an alarm on an older vehicle than a new one, provided you care about the older vehicle and don't want to go through the hassle of replacing it. The old ones are easier to steal for a joyride, and that's why most auto theft happens around here. But the really easy ones are the ones where the keys are accessible: either in the vehicle or the house. I keep any car keys left at home well hidden if I'm away without the car.