today I spent some time getting my MCP3204 working. It´s a 4 channel ADC with an SPI Bus.
I connected the MCP like this:

#define SPI1_CLK  18  // D -> Clk   -> Pin 11  
#define SPI1_MISO 19  // Q -> D Out -> Pin 10     
#define SPI1_MOSI 23  //   -> D In  -> Pin  9    
#define SPI1_SS   16 //    -> CS    -> Pin  8   

It´s working as expected as far as I don´t use any display stuff from the M5 Stack.

As soon as I ad the following to my code:

 M5.begin(); // true, false, false);
 M5.Lcd.printf("TEST");

things getting wired. The readings of my MCP3204 are wrong.

I convert the AD values to temperature vales since I use some BBQ sensors. And the results are about 2 degrees wrong when adding the M5 Display.
Example: Reading temp without Display -> ~21° and with display ~23° or sometime ~17°
I can do some addition checks with the plain AD values to get a better understanding.

But anyway ... What´s wrong in using the SPI bus for a second device?
Are there any drawbacks i don´t know? Any issues?

Maybe someone has some explanation for this topic.

Dominik