There are numerous heroes that have area of effect spells and in game it does not give much of a description for these spells. What is generally not described is that these spells usually have either a damage cap on the maximum damage these area of effect spells can do in total or a unit cap on how many units said spell can hit.
For example, lets take carrion swarm, one of the Dreadlord spells. In game as of this patch (and sometimes this and the max damage/unit cap is changed) the explanation simply says this:
If you ever used this spell you may have noticed that sometimes when you use this spell on a clump of units not all of the units always take damage. That’s because there is a max damage cap built in to this spell that is not explained in game. This spell (as of the current patch) can do a max damage of 400 at level 1, 700 at level 2, 1200 at level 3 in TOTAL DAMAGE. The spell cannot do more damage than what I listed above. So sometimes you will see a unit get hit by the spell, but if the max damage cap has already been achieved, it will not do any damage to that unit.
There are also sometimes similar caps on the amount of units that can be hit by a spell. Like the Warden’s Fan of Knives. In game it simply says:
Sounds awesome right? Well it is, but not as good as it originally sounds. Fan of Knives can only hit 4 targets level 1 and 5 targets on level 2 and 3 max (as of this patch). So while you may use it within range of 10 units, only 4 or 5 will be hit depending on the level of the spell.
Some units also have similar caps, like obsidian statues can only heal health and mana of a max of 6 units at a time.
Where do I see all these caps?
Liquipedia keeps an updated list of heroes current spells as well as extra information concerning about each of these spells, such as if there is a max damage cap or unit cap.
Go to this link https://liquipedia.net/warcraft/Hero select a hero and you can learn all about the spell in detail including if it has a max unit or damage cap. These numbers can change patch to patch and liquipedia and its editors keep this list up to date.