Your comments all assume the companies involved in this relationship are all rational and good actors, and illustrate the double-standard companies have. They expect complete loyalty and "reliability" (your definition), but don't offer employment contracts to reflect this; in fact it's almost universal that employment "contracts" have an "at-will" clause. To me, that is a statement by the employer that they aren't interested in a long term relationship, and I shouldn't expect one.

Not to mention that in companies of any size, the reality is that "the company" is actually a lot of smaller companies run by a middle-manager who himself has his own (frequently corporate-political) motives for doing things, and is fickle.

To act rationally on that premise isn't unreliable; it's self-preservation.

What if the company, after some time trying to work a project out, decides that it is more profitable to shut the project down - does that also make the company unreliable? Where I want to get is that interests evolve and things change. If in two-three years you want to check something out, that does not make you unreliable, but just normal part of the daily business life.

I wouldn't worry as much about an arbitrary "unreliable" designation as I would about my career stagnating because I haven't learned anything new in a year. A few years is all the growth potential most jobs offer. Once you've solved all the problems, do everyone a favor and hand your tasks off to someone more junior and seek out greener pastures.