Our Gemara on Amud Aleph warns of using excessive praise of G-d during prayers:

⁦Rabba bar bar Ḥana said that Rabbi Yoḥanan said: With regard to one who excessively declares the praises of the Holy One, Blessed be He, his fate is to be uprooted from the world, as it appears as if he had exhausted all of God’s praises. As it is stated: “Shall it be told to Him when I speak? If a man says it, he would be swallowed up” (Job 37:20). The Gemara interprets the verse as saying: Can all of God’s praises be expressed when I speak? If a man would say such a thing, he would be “swallowed up” as punishment.

There is a similar Gemara in Berachos (33b), which states:

⁦With regard to additions to prayers formulated by the Sages, The Gemara relates that a particular individual descended before the ark as prayer leader in the presence of Rabbi Ḥanina. He extended his prayer and said: God, the great, mighty, awesome, powerful, mighty, awe-inspiring, strong, fearless, steadfast and honored.

⁦Rabbi Ḥanina waited for him until he completed his prayer. When he finished, Rabbi Ḥanina asked him: Have you concluded all of the praises of your Master? Why do I need all of this superfluous praise? Even these three praises that we recite: The great, mighty and awesome, had Moses our teacher not said them in the Torah and had the members of the Great Assembly not come and incorporated them into the Amida prayer, we would not be permitted to recite them. And you went on and recited all of these. It is comparable to a king who possessed many thousands of golden dinars, yet they were praising him for silver ones. Isn’t that deprecatory? All of the praises we could possibly lavish upon the Lord are nothing but a few silver dinars relative to many thousands of gold dinars. Reciting a litany of praise does not enhance God’s honor.

Halakha Lemaase, Rosh, Rashbah and Rav Hai Goan say this restriction is only for public prayer and not private. However, the Rambam seems to maintain that it is even prohibited in private prayer (Tur OH 113). This position of the Rambam is especially understandable in that he holds that any metaphoric descriptors of God having material or emotional qualities is tantamount to heresy.  See Yesodei Hatorah chapter one and Laws of Repentance 3:7, especially Raavad’s critique. Thus, what difference would it make if the prayer is in public or private? One way or the other it represents a disrespectful way of describing God.

The Shita Mekubetzes modifies Rambam by saying that he is only referring to descriptors of God’s essence. However there is nothing wrong with being excessive and praising God’s deeds and miracles.  Given the reasoning above, this makes sense.

We must ask ourselves however, what possible defense could there be to the position of the Rosh, Rashbah and Rav Hai Gaon? Why would such excessive praise of God's essence be permitted privately. Should not the same reasoning apply?

Penei Yehoshua offers the following answer. When one is in formal prayer, one must be formal before the king. However, during private prayer, there might be aspects of God’s blessings that need to be tapped into. Therefore, various kinds of praises of His essence are necessary in order to draw down that particular blessing. This is why we find that sometimes Moshe does not actually use the full 13 attributes when asking for forgiveness, such as by the sin of the spies. This is because he knew which particular attribute he needed to draw down from God.

I really enjoyed this explanation because it also offers a deeper understanding of the essence of prayer. Somehow by mentioning and meditating on a particular essence or quality of God, even if it is only our human understanding, it activates and draws down that power and blessing. That is a worthwhile thought to contemplate.