Executing command on Plink command line fails with

Goal: connect to AntMiner via SSH, and send poweroff command.

command: plink.exe -v -t -ssh antminer -l root -pw xxx poweroff

Connecting to 10.0.1.11 port 22  
We claim version: SSH-2.0-PuTTY_Release_0.70  
Server version: SSH-2.0-dropbear_2012.55  
Using SSH protocol version 2  
Using Diffie-Hellman with standard group "group14"  
Doing Diffie-Hellman key exchange with hash SHA-1  
Host key fingerprint is:  
ssh-rsa 1039 xxx  
Initialised AES-256 SDCTR client->server encryption  
Initialised HMAC-SHA1 client->server MAC algorithm  
Initialised AES-256 SDCTR server->client encryption
Initialised HMAC-SHA1 server->client MAC algorithm  
Using username "root".  
Sent password  
Access granted  
Opening session as main channel  
Opened main channel  
Allocated pty (ospeed 38400bps, ispeed 38400bps)  
Started a shell/command  
sh: poweroff: not found  
Server sent command exit status 127  
Disconnected: All channels closed

So the connection is working fine, it's the sending of the command that goes wrong. Here's what SHOULD happen: (sent the command manually this time)

command: plink.exe -v -t -ssh antminer -l root -pw xxx

Connecting to 10.0.1.11 port 22
We claim version: SSH-2.0-PuTTY_Release_0.70
Server version: SSH-2.0-dropbear_2012.55
Using SSH protocol version 2
Using Diffie-Hellman with standard group "group14"
Doing Diffie-Hellman key exchange with hash SHA-1
Host key fingerprint is:
ssh-rsa 1039 xxx
Initialised AES-256 SDCTR client->server encryption
Initialised HMAC-SHA1 client->server MAC algorithm
Initialised AES-256 SDCTR server->client encryption
Initialised HMAC-SHA1 server->client MAC algorithm
Using username "root".
Sent password
Access granted
Opening session as main channel
Opened main channel
Allocated pty (ospeed 38400bps, ispeed 38400bps)
Started a shell/command
root@antMiner:~# poweroff

Broadcast message from root@antMiner (pts/0) (Thu Jan 25 19:23:19 2018):

The system is going down for system halt NOW!
root@antMiner:~# Server unexpectedly closed network connection
FATAL ERROR: Server unexpectedly closed network connection

Anybody have any idea WHY the sending of the command "poweroff" over plink fails?

This is partly covered in this question:
Script via Plink in .bat behaves differently

So you can particularly try which poweroff in a normal session, to see where poweroff resides (can be /sbin/poweroff). And then use a full path in your plink command-line.

As your command-line does not work even with the -t switch, your SSH server must execute a command in "exec" channel (used when a command is provided on command-line or using -m switch) differently than in a "shell" channel. This is rather unusual.

You can force plink to use "shell" channel (like in an interactive session) by using an input redirection: