The Problem with Pietism

It commends a half-religion, which is another name for idolatry and paganism
The big problem with pietism is simply that it is only a half-faith. It tries to receive God's promises as from the Almighty, and yet it does not receive God's commands as from the Almighty.

So pietism is a half-faith in a half-God that leads to a half-Christian life. The problem with that is that the Bible tells us that half-faith is false religion, and that a half-God is actually and idolatrous caricature and not the true and living God, and half-obedience is disobedience.

It contradicts the most basic Scriptural teaching about the Christian life
When we learn that 1Cor 10:31 intentionally uses the most mundane things--eating and drinking--to instruct us in what the divinely blessed life is, then we are prepared to brush teeth, sweep floors, and yes proclaim the gospel and sing praise... all to the glory of God.

It contradicts the most basic elements of what we confess Scripture to teach
Our catechism's first question and answer encapsulate this well: the chief end of man is to glorify God and enjoy Him forever. This is not just our "primary purpose" as some contemporary wordings have tried to say it. It is the "controlling purpose"--the chief that commands the purpose--in absolutely everything that we do.

So, God has not given us a single thing to do, except that it must be done unto His glory. And, God has not given us a single thing to do, except that we must do it in enjoyment of Him.

It confuses poor theology students into downplaying the importance of exercises of piety
Now, please be cautious. Do not throw out biblical piety--and a strong devotion to the exercises of piety--as you reject an unbiblical pietism. There is hardly anything that God commands more than the exercises of piety. The first four commandments--and especially the first and the fourth--strongly stress this piety.

Communion with God continually and daily and weekly, in the assembly and family and privately, are everywhere commanded in the Bible. If these are not the life-breath of our existence, and the hinges upon which our days and weeks and lives turn, then we shall not be pious in any activity whatsoever.