Cogs and holks are just variants on the same basic ship. A cog begins at 45 lasts, and can upscale in size like the snaikka and crayer. The holk isn't available, until you have upgraded your shipyard through money spent in construction and repair. It begins as a 55 last model, and ends as a 70 last model. You can acquire them earlier in the game from the pirates, but only as the "improved" models. This cuts their cargo capacity.
Unlike the earlier ships, the cogs and holks can carry weapons without being extended. A cog will carry two large weapons, and at final extension, four large and one small weapon on a side. A holk will carry five large weapons on a side, or ten altogether, fully extended.
Cogs are slow, and can easily be outrun by a crayer, or by a holk. A holk is a fast as a snaikka, no better. For some reason, the freesailing pirates (those not found in the tavern missions) prefer them. At first the ships will be extended once, but rapidly progress to second extension models, good for carrying only limited amounts of cargo. Frankly, I usually just capture them early on and auction them off to the AI competition. While a 70 man holk fully extended can carry 5o lasts, and mounts an impressive broadside, it cannot take out a crayer due to the faster manuverability of the smaller ship. I have even sunken a holk with a snaikka. So, for bulk goods, which don't require fast movement, they are acceptable, but really aren't worth running overall.
As throwaway ships, for a massive assault on a city or a pirate nest, the cogs or holks have their uses. But overall, they just do not cover their expenses adequately.