Gamers Nexus did a very in depth review of the Steam Machine [1], which includes a comparison to a build yourself similar machine.
The result is that for about 70 dollars less you can put together a somewhat more powerful PC than the Steam Machine, but not for that form factor, it would still be bigger.
IMO, the Steam Machine is not a bad purchase if you are in the market for that type of product.
You can't build a machine which is as powerful, small, quiet, and cheap, nor can you take for granted that a machine you build can have a controller that can wake it from sleep, or which has HDMI-CEC (both are possible, but take extra work or hardware). You can rather easily build a machine with multiple of those attributes, but you'll have to pick ones to sacrifice in the name of the others.
Those will be cheaper and/or more powerful. If that's all you care about, then the Steam Machine probably isn't worth the price or wait.
But they'll also be bigger, louder, probably not support HDMI-CEC, may or may not support waking via bluetooth, and it's a gamble whether they'll support Linux as well. Both are totally valid options, it just comes down to which tradeoffs you prefer.
The "boring" business boxes from the usual suspects are usually well supported by Linux, even if HDMI-CEC and BBT wake might not be priorities for the manufacturers. The hardware is so unimaginative it's almost an Intel or AMD reference design.
The Steam Deck itself doesn't have an HDMI port, but the official dock does, and that does support HDMI-CEC (support was added to SteamOS a couple years ago). The Steam Deck also does support controller wake from sleep (added in the past year or so); I've actually seen people complaining about their Steam Decks waking up when they didn't realize that feature existed.
And all of the reviews I've seen about the Steam Machine talk about how well both of those features work.
It does not. Source: have a dock. Latest firmware installed from deck. And because the current Steam controller is a recent release, I had to try a whole mess of third party controllers before settling on PS5 controllers because everything else I tried had Bluetooth pairing issues / disconnects as you approached four controllers.
SteamDeck as a handheld is great plus or minus a few nits baked onto the power / battery life choices Valve made. SteamDeck -> TV and SteamDeck -> USB-C KVM are both workable, with caveats. I had hoped we would see the bug fixes you describe before the Steam Machine release. Alas, no.
I will say that the Deck has less than stellar bluetooth reception in my experience too. I settled on an 8bitdo controller because my XBox Elite couldn't stay connected from across the room. The Steam Machine has a dedicated antenna for Steam Controllers though.
Read the other replies, and i’ll restate that you are indeed confidently incorrect: i have been using this feature since the update that added it. i use the steam controller and i am using the original valve dock.
Especially since, afaik, you plug it in and it just works. No messing around with installing operating systems, setting up users (aside from signing into steam) or anything. It's essentially a console that plays PC games, but it's also a PC for the purposes of upgradability and ability to do other, non-console stuff with it
Compared to an average prebuilt? You can probably find large tower PCs at a lower price, but they'll likely have a low quality motherboard or power supply.
Compared to an average prebuilt that ships with Linux? Absolutely
The result is that for about 70 dollars less you can put together a somewhat more powerful PC than the Steam Machine, but not for that form factor, it would still be bigger.
IMO, the Steam Machine is not a bad purchase if you are in the market for that type of product.
[1] https://m.youtube.com/watch?v=66QzlDewigE