It is not effective for new apps because usually they don't have keywords;
If you don't have your keywords, it will be difficult to find the best one (but you can use ASO services to solve this little problem);
Sometimes you can drop out of the keyword rank because of the changes in google algorithm.
Keywords promotion is 100% effective and after boosting with keyword installs your app will get higher in ranks, more visibility and more chances for users to find and download your app.
Campaign for keywords installs is a good way to combine a lot of different particular keywords and run it at the same time;
Keywords are more effective for apps, which already have enough installs;