1)A PLC can be programmed by the user by a method he already knows
A PCU is programmed by the manufacturer(s representative) by a method the end user has no interest, desire ot willingness to learn.

2) A PLC is very low in customer support cost since the customer basically does all himself and - since the PLC "language" is universal you can charge for customer support
A PCU must be programmed as customer support AND the customer is guaranteed to tell you it is your fault i.e. you can not bill him.

3) A PLC can be sold at 'cost plus profit' since no money for free customer support has to be built in.
A PCU must bhe sold for 'cost plus profit plus cost of customer support' and when calculated correctly will be more expensive than a PLC

re 3) If you up front tell your customer "if ANYTHING has to change for WHATEVER reason, it will cost you" you may be able to get around 3) but the statement does not sound as good sales promotion to me.

4) There IS a market for PCUs, but not as 'PLC replacements'. The market exist where a lot of PLCs with identical programming (save a parameter or two) will be employed e.g. as a control unit for a machine that is built identical in a reasonably high number.

Erik