What is the effect of using the ESP12 WiFi module without the metal cover?
6\$\begingroup\$ Probably a lot of EMI, possibly enough to affect the Wifi/BT signal. \$\endgroup\$– Ron BeyerMar 2, 2020 at 18:24
2\$\begingroup\$ FCC violations, you need the cover on \$\endgroup\$– Voltage Spike ♦Aug 6, 2022 at 20:07
The metal cover is placed there to mitigate EMI.
However, in the ESP8266 hardware design guidelines, there is no mention of the necessity of such shield. So assuming that the ESP-12 has a decent design and proper grounding, the WiFi performance should not be impacted too much.
3\$\begingroup\$ Wifi performance of the esp8266 may not be impacted but it may emit EMI and interfere with other devices. All of the ESP8266 modules I've seen that have FCC markings have a metal shield. \$\endgroup\$ Mar 30, 2022 at 23:25
The only source of EMI inside the module (other than RF itself) is crystal oscillator, and it is already very close to RF traces. Removing the cover will not affect the signal. It can, of course, increase interference with EMI-sensitive electronics nearby, but it will be in completely different band (MHz vs RF's GHz).
Some ESP modules have separate flash chips. Switching noise of their pins also can contribute to EMI, but its frequency will be even further away from RF signal than system clock.
Also, to address multiple references to FCC certification in other answers and comments, I would like to point out that the whole purpose of these devices is to emit electromagnetic radiation. By their very nature they will interfere with other devices operating on the same frequency.
For this reason the only FCC certification they carry is part 15, subpart C, which is called "Intentional Radiators". That certification is mostly to ensure the device will not stray away from its designated band and allowed power. You can download other ESP certifications from Espressif site, if you are curious.
In short, while removing the shield will indeed increase EMI at system clock frequency, it will not break FCC requirements because the module does not have a relevant certification to begin with. This is not to say it is a good thing to do, though. Technically, it won't be the same device anymore, so it will lose all the certifications anyway.