Atmospheric but muddled.
With Drums in the Marsh, the Tyranny of Dragons AL season takes a detour to the Twilight Marsh for an encounter with lizardfolk.
There's a lot to like here, staring with an extremely atmospheric setting. The module does a great job of emphasising a potentially terrifing trek across the marsh, complete with pounding drums, driving rain, a river voyage, relentless lizardfolk pursuit, and dangerous marsh creatures. The finale rather unusually (for this AL season) offers players a lot of opportunity for creative, aggressive or diplomatic approaches. The lizardfolk are well used, with a lot of emphasis on how they work as teams and how they make use of their natural environment to increase their threat. These are all really strong points.
But the module definitely needed an edit. The opening "investigation" phase feels tired and obligatory. It's clear from the first words of the module that lizardfolk are the villains, but the first act nevertheless goes through a tiresome series of "what could possibly have left these clawed footprints" site-based investigations. When I ran the module, I skipped this first act entirely, starting in media res with the party already travelling down the Trank River by boat.
Another serious defect is the lack of an overland map. Much of the module consists of a wilderness journey through the Twilight Marsh, with encounters at various points along the way, but the module includes no large-scale map of the area showing the relative positions of these encounters, nor does any good detail map of the area exist in other Forgotten Realms material that I'm aware of. It makes it hard for players to make sensible decisions about their progress or engage with the exploration portion. (To be fair, in its original AL format, the time constraints of an AL game would normally prevent players making this level of decision anyway, but it's still a noticeable and jarring omission.)
The module also fits poorly into the larger Adventurer's League season, using none of the existing worldbuilding around Phlan but existing in its own standalone space. A DM of reasonable sharpness can bridge this gap - or alternatively, it may make it more attractive to DMs looking for a lizardfolk-themed one-shot, as the whole module could easily be dropped into any marshland in any campaign setting. It also regrettably repeats some themes from the immediately preceding adventure, "The Scroll Thief", featuring another trip to the Twilight Marsh and the involvement of another black dragon, after the players have possibly already just dealt with those exact same plot elements.
Lastly, it suffers (as many AL modules do) from possibly not meeting player expectations regarding the finale. Events in the module have largely been set into motion by a villain who does not actually appear within the module scope. The later module "Raiders of the Twilight Marsh" picks up some of these plot threads, but player expectations need to be managed or they'll be expecting a fight that never comes.
Summary: A good highlight of lizardfolk, easy to use as a generic marsh-themed adventure, but a poor contribution to the surrounding Adventurer's League season burdened with a dull first act.