While the possibility of a crankcase explosion is quite low, the result is too dangerous for this procedure. Vacuum leaks into the crankcase can be tested by blocking the PCV and fresh inlet, and then connecting a vacuum gauge to the dipstick tube and checking vacuum. If there is a vacuum leak into the crankcase and there are no gasket or seal leaks in the crankcase, then the leak will show up as high crankcase vacuum. Carb cleaner is not very effective at finding leaks on the underside of manifolds and in vacuum lines that run through wiring looms. Short of soaking everything under the hood, you just aren't going to find some vacuum problems with carb cleaner.
Oil is another alternative to carb cleaner.
The idea is the oil is thick enough to seal the vacuum leak for a short time. This is a good technique if you have found a leak at the top of a manifold gasket by using carb cleaner and it seems a significant leak that you can test for size by using oil. However, oil is difficult to apply in many places and has limited uses. Propane is probably the best alternative to the above techniques. A fire is usually short-lived if one occurs and is much less likely.