I would say that one fundamental reason why Taylor's theories could not be effective motivation tools for engineers is because this is not the paradigm's expressed purpose. Taylor was not concerned with employee motivation. Rather, he sought to increase efficiency and productivity with an intense approach to ensuring results as being critically important. The idea of specializing each job to a degree where workers are not able to exercise autonomy and differentiation of talents is something that might not be especially appealing to engineers who wish to create their work setting differently and better. Taylor's theory is driven by the idea that efficiency and results are more important than worker morale and the development of new and diverse exploration of talents. For engineers who are concerned with what might be as opposed to what is, Taylor's theories might be a challenge because they give little in way of providing a sphere or zone of influence where creative empowerment is evident.
Taylorism refers to methods of management developed and promoted by F W Taylor in early twentieth century. These methods among others include provision of payment of incentives in form of additional pay for additional work output to motivate workmen to give greater output. While adoption of this approach to motivation resulted in substantial improvement in productivity, it has many limitations which makes it inappropriate to be used as the only method for motivating labour. Here, it must be pointed out that Taylorism itself recognizes presence of other factors influencing motivation though it does not elaborate upon them.
The main limitation of using monetary incentives linked to work output as motivator is that it can only be applied to kind of work in which work output of individuals can be measured with reasonable degree of accuracy. This is quite easy in manual work, performed by individuals working in isolation. However, for many types of work the output or results cannot be measured easily, particularly in short term. For example, output of all work involving application of intellectual capabilities - for example, work of a designer or a manager - cannot be measured easily. Also often it is difficult to determine contribution of individuals when the work involves working as teams.
Also, in the world today, when the competitive forces of the labour market and the government regulations regarding conditions of employment, have ensured a fairly higher level of prosperity to labour in general, the relative motivating power of money has diminished. Today, a company cannot expect to find people to work with them unless they are able to offer them fairly good basic financial remuneration and job security. Now companies must find additional ways of motivating employees that appeal to their need such as social, ego and self actualization needs.