IMHO, I'd stick with the apps that are preinstalled on your phone first before trying an alternate. For example, use the native texting app before experimenting with others. Stick with the HTC camera software.
The apps installed by HTC are designed to work with your M8. Other apps have to be designed to work with most other phones and could have quirks that are not fully compatible with your phone or your software version.
Then, if you want to try other apps, play it safe and only get apps from the Google Play Store.
Read the reviews carefully for each app and avoid any app rated less than 4
Check the reviews that were written after the latest update for the app (newest), for your phone.
Then do a Google search and look for reviews of that app in places other than the Play Store.
Good Luck!