The specific anticholinergic effects you mentioned of Benadryl and other 1st generation antihistamines are very modest. The level of urinary retention caused by these drugs is nothing like that of something like imipramine. It can affect the peristalsis of the GIT but generally most people don’t experience issues with that. Xerostomia and dry eye can cause problems if prolonged.... But considering a cold usually lasts no more than a week, such side effects are usually considered negligible unless there is a patient specific contraindication. There are some populations where 1st gens are contraindicated (such as the elderly, pregnant, those on certain medications). All medications have potential side effects, their benefit should be considered relative to the side effects.