I think one important factor is the types of algae growing. In most cases (except for cyanobacteria) algae are found in the kingdom protista, a "grab-bag" of organisms that don't fit precisely into the animal or plant kingdom. For example, included in the protista kingdom are multicellular and unicellular organisms, eukaryotic and prokaryotic, and organisms that range from autotrophic to half-autotrophic/half heterotrophic to heterotrophic. Because of this, each variety of algae has entirely different triggers/niches.
This is one of the reasons that I have doubts about allelochemistry playing too large a role in plants' competition with algae. It seems that a plant that is effective at combatting one type of algae would have a hard time with another, as allelochemistry works to block specific enzymes. Of course, the allelochemical could target a single enzyme that is common to all photosynthetic protista, but this seems unlikely IMO when you take the variety of protista into consideration.
Hornwort is the plant about which I have seen the most research relating to allelochemistry. Has anyone noticed whether hornwort combats all algaes equally effectively in a broad range of conditions, or is it more effective against a specific type?