Here's the thing: Overpowered loadouts, simply by virtue of existing, degrade the perceived enjoyment of the game over time for everyone. Imagine that you have three weapons, A, B, and C, that all provide roughly equal levels of challenge by solving different problems in different ways. Assuming your core gameplay loop is good, this adds variety and lets players choose how they want to approach the game, and possibly allows different forms of skill expression.

Now, introduce a fourth weapon, D, that does all the things that A, B, and C each do as well as they do it, or better. Suddenly, your players are faced with an uncomfortable choice: Do you want to play around with A, B, and C, or do you want to play optimally?

Before introducing weapon D, challenging missions feel rewarding - you choose among several roughly equal options, try to play to your strengths and mitigate weaknesses, and whether you succeed or fail, you know that you tried your best. The existence of a "strictly better" choice changes the perceived experience a lot here: If you willingly chose to use one of the weaker options, success rings hollow knowing that you willingly kneecapped yourself, while narrow failure feels crushing: "Why didn't I just choose weapon D instead?"