To say that participles function as adjectives means that they relate to nouns very much as adjectives do. It does not mean flatly that any participle may be used exactly like a typical adjective.
Some -ing forms - such as 'interesting, exciting, boring' - are considered true adjectives (known as 'participial adjectives'), while others - e.g. 'writing, eating, deciding' - are not, even though they may often precede nouns in such a way as to make them seem so. The key point is whether we may grade them with an adverb such as 'very': very boring is acceptable, while *very writing is not.