Why are Python multiprocessing Pipe unsafe?

I don't understand why Pipes are said unsafe when there are multiple senders and receivers.

How the following code can be turned into code using Queues if this is the case ? Queues don't throw EOFError when closed, so my processes can't stop. Should I send endlessly 'Poison' messages to tell them to stop (this way, i'm sure all my processes receive at least one poison) ?

I would like to keep the pipe p1 open until I decide otherwise (here it's when I have sent the 10 messages).

from multiprocessing import Pipe, Process
from random import randint, random
from time import sleep

def job(name, p_in, p_out):
    print(name + ' starting')
    nb_msg = 0
    try:
        while True:
            x = p_in.recv()
            print(name + ' receives ' + x)
            nb_msg = nb_msg + 1
            p_out.send(x)
            sleep(random())
    except EOFError:
        pass
    print(name + ' ending ... ' + str(nb_msg) + ' message(s)')

if __name__ == '__main__':
    p1_in, p1_out = Pipe()
    p2_in, p2_out = Pipe()

    proc = []

    for i in range(3):
        p = Process(target=job, args=(str(i), p1_out, p2_in))
        p.start()
        proc.append(p)

    for x in range(10):
        p1_in.send(chr(97+x))
    p1_in.close()
    for p in proc:
        p.join()
    p1_out.close()
    p2_in.close()

    try:
        while True:
            print(p2_out.recv())
    except EOFError:
        pass

    p2_out.close()