I think it depends on the scope of the mission. For a small to medium, fixed location (non-mobile) business, I would say that it could be mission critical. For larger operations or anything requiring a mobile workforce, I would say no.
I work for a small, extremely mobile business and for our office operations, it works great. Speed of development is pretty awesome and apps work reasonably well with a decent user experience. In fact, we will probably use PowerApps as our primary way of dealing with company information in the near future. We aren't a "millions of rows of data" level business yet, but we are at tens of thousands and, with some creativity, have been able to keep performance fast enough for our purposes. There have been some bugs/issues that took us down for a couple days, but I trust that those issues will be fewer and farther between as the platform matures.
On our field side, not so much. We have been trying to use PowerApps as a maintenance tracking solution and, consequently, as a part of our safety culture on the leading indicator side. Our employees are in a high-hazard industry working with heavy equipment, so this is an important aspect of our business. As discussed here, even though our apps have fully functioning offline capabilies, we are unable to open apps without an internet connection. When our supervisors have to leave the location to record maintenance, sometimes driving 20-30 miles offsite, we are removing one safety aspect to produce another. So to say that I don't trust it currently for human safety is quite accurate. At this point, using an Excel-based form that I could import into our database would work better, so PowerApps isn't even a good form replacement in this scenario.
PowerApps is a fairly young platform and in some ways I think they are still trying to find their identity. If they could develop a fully-functional mobile experience (online or offline use) and make it a little easier to bridge the learning gap on how to handle larger data sets, I think it would be quite a bit closer to being fully ready for mission-critical uses. And for businesses with a more localized scope, I think it has the potential to be ready now.
