According to purists, a few adjectives have no comparative or superlative modifiers. These include eternal, fatal, incessant, maximum, minimum, optimum, complete, perfect, and unique.
(U) In fact, most adjectives cannot be compared because their meaning is too technical or specific, or they name a quality that cannot exist in degrees. Examples are diocesan, antinomian, aquatic, graven, and electromagnetic.
(U) Adjectives expressing qualities are never compared when used in their strict sense. When used in a modified or figurative sense, however, they have been and can be compared.
(U) Actually, respectable writers have used qualifiers through the centuries for many of the so-called absolutes. The U.S. Constitution, for example, includes the phrase "to form a more perfect union." Here "more" is used to mean "more nearly."
(U) Therefore, if you are using these adjectives in a figurative or modified sense, go ahead and modify: Sandy has a more complete understanding of Hegelian philosophy than anyone else I know. If you are using them in a literal sense, do not modify: There are no perfect humans.
(U) See the entry for unique.And we discover that "accept means `receive with consent' or `approve of': He accepted the nomination. He could not accept the situation. It should never be confused with the verb `except'." Feel free, though to confuse it with the preposition except.
It is, however, very reasonable about the passive voice: "(U) While the active voice tends to be shorter and more direct, there are good reasons to use the passive voice. Do not use a hard-and-fast rule (`Avoid the passive voice') but consider each case carefully before deciding which to use."
Lots more to dig up here.