Brown widow spiders are venomous, but they're considered much less dangerous than the black widow. Although brown widow venom is just as toxic, the brown widow spider injects a much smaller amount of it. Also, only the adult female brown widow spiders bite. Immature and male brown widow spiders don't bite at all.
Yeah, I mainly use it when my son and I are hiking and he asks 'The hell is this weird thing?', I remind him I'm no botanist and pull up seek. It's a pretty cool app. Just looked, surprised the reviews are 3.3, never had any problems with it.
I might be biased because I identify spiders for a hobby and these apps put me out of work.
But there are real problems with them. They work fine approximatly 80% of the time and if you just want to learn some fun facts about local insects and plants, that's great. Worst thing that can happen you learn something interesting about the wrong thing.
However, you'd never want to trust an app for picking mushrooms that you're planning to eat. (I hope you don't, if you do, don't!). That's very dangerous.
Spiders are somewhere in the middle. And if you're actually concerend that a spider might be medically significant, I would not trust an app for that.