These mirrorless cameras have a number of advantages compared to a DSLR, such as lighter weight and a smaller size, but they also have a number of disadvantages, the main being sensor size and lens availability. The imaging sensor in the mirrorless cameras is much smaller than in traditional DSLRs, which translates to increased noise, or digital grain most prevalent in low-light situations. And they almost all use different lens mounting systems, meaning less options for choosing different lenses.
Megapixels are overrated in the camera industry. Camera manufacturers will often boast about their insanely large megapixel counts. The reality is that after a certain point, more pixels create more noise because only a certain amount of data can be put on a sensor. More megapixels allow you to print bigger, but if you aren’t going to be printing your photos, you don’t need to put too much thought into it. Otherwise save money on megapixels, and focus on lenses. Here’s a handy chart that compares the number of megapixels to print size.