No concierge means nowhere to stash your bags if you arrive early or leave late. If you have a problem with your accommodations at a hotel, someone will usually come to your room quickly and even change your room if the problem can't be fixed.
At an Airbnb, you're at the mercy of your host, who may or may not be responsive. During my last Airbnb stay, the lamps in the apartment were broken, and the shower didn't drain. I told the host, who thanked me for letting him know but did nothing to fix the problems during the week I was stuck there.