Calling a product a piece of shit is perfectly acceptable behavior, it often is a reasonable and accurate opinion about a product and definitely meets the standards of the "level of decorum" expected for customer product reviews.
Customers shouldn't be absolved of having a minimum level of decorum, but that minimum is at the level of (a) personal insults at specific people ("Bob in customer support is a piece of shit" wouldn't be ok but "your customer support system is a piece of shit" is fine) or (b) libel, as decided by the legal system.
For direct criticism of a product or service literally anything that I imagine would be above that minimum level of decorum. Insulting people is not ok, but insulting products definitely is.
Customers shouldn't be absolved of having a minimum level of decorum, but that minimum is at the level of (a) personal insults at specific people ("Bob in customer support is a piece of shit" wouldn't be ok but "your customer support system is a piece of shit" is fine) or (b) libel, as decided by the legal system.
For direct criticism of a product or service literally anything that I imagine would be above that minimum level of decorum. Insulting people is not ok, but insulting products definitely is.