The OE fob just adds another button for factory starter, which isn't anything special.
You can get 3x lock start from the OE fob with idatastart units, so you can get both an aftermarket remote start remote for longer range situations and can utilize your OE fob.
Major downside of 3x lock start with OE fob is range. You're limited to the range of your OE fob. You also can't have 2-way confirmation through the OE fob. My idatastart in my '03 6 is over 2000ft line of sight range with 2-way confirmation. Can't live without it.
You say warranty? Are you alluding to the dealer saying something about voiding your warranty by having a non-OEM remote start installed? Because under the Magnusson-Moss act, they can't deny your warranty work because of an aftermarket component installed in the car, especially if it is professionally installed by an authorized dealer/installer of that product. If they are, it's a major scare tactic they are using to squeeze you into buying theirs.
No, I meant to say "good," because I don't think the aftermarket is truly and wholly "better."
The additional button is exactly what makes it special: not having to carry around a second fob.
Extended range is nice, but how often is one within a 2000 foot line of sight range to your vehicle? A line of sight that is free of other signals, obstructions, and objects. Unless your remote start works off a cellular network, that is. There's not a place in or out of my city I've been where I have a line of sight to my car greater than the OEM fob.
2way confirmation is nice, which I believe is offered with the MMS app. Downside is the subscription price.
I say warranty as in, the Mazda accessories warranty. Most aftermarkets offer their own warranty as well, but an OEM product will cooperate better with the manufacturer than an aftermarket will; MM act or not.
To each their own with their choices. I like the additional alarm and features my Compustar offers, but with how I use mine, I wish I'd've just financed off the OEM remote start. The aftermarket is so stupidly sensitive, I accidentally set it off once a month - and that's after a year of usage.