Linux users have been over-selling this device because they view it as a gateway for new users to Linux, and because it will change the statistics they can rattle off about Linux users. They don't care if there are better options, this is the product they glorify despite all its faults.
No exclusive games and limits available games by being underpowered and not running Windows well. Games that run may not run good at all. Some games with kernel-level anti-cheat systems (like Vanguard or ESEA) are not supported or lost support. I don't know about you, but it's usually a particular game or games that gets me to buy a console.
2-6 hours of playtime between charges. 2.5-3 hours to charge! -Does not support fast charging! The battery also discharges over time when not being used.
It gets hot! -You can't simply put a pillow under it to support its massive weight without affecting cooling. -And it's freaking heavy!
1280x800 is like going back to pre-HDTV era screens and isn't ideal for all games. The screen is a common complaint as one customer indicated it's not on-par with phone screens, with horrible touch sensitivity, while others don't like the bevels. OLED models may have mura (inconsistent pixel brightness).
Sometimes launchers from the developers don't allow bypassing from Steam. The launchers can be large and unwieldy on its small screen.
Can't download during screen lock (needs babysitting). -Nintendo Switch can download while you're away!
There is no Game pass or Microsoft store compatibility. Microsoft makes some of the best games of their genres.
People are complaining about the controls being uncomfortable (particularly the back ones) and the keyboards sucking. No control keys or Esc for the on-screen keyboard? The D-pad sucks for diagonal controls, and the sticks are slippery. Some people are using the trackpads to compensate poor / inaccurate diagonal controls.
Valve hasn't up streamed their customizations for the device into the mainline kernel making other Linux distros suffer issues.
Linux issues are carried over, like Bluetooth pairing, and waking from sleep. It's reported that 20% of the time it will crash on wake-up.
If you haven't played a game in a while, its license may expire making a game stop working, or you may run into issues with saved games.
A single USB-C port? -Isn't it supposed to be like a PC?
Someone reports having to re-download shaders every time they turn off or reset the device, another claims they often end up tinkering instead of playing.
Inconsistent parts. -You might get the noisy 'screechy' fan version for example.
$400 for a portable console with no exclusives is insane! -Compare the price of smart phones at ~$100 which can play games, a laptop, or a home-built desktop that could play more games.
It's repairable, but you may have to source the parts from Valve.
Major issues I've seen with the dock are a whole other article!
you can do modding on Linux and on a steam deck. you can also use keyboard and mouse on a sream deck