Horde are usually point effective, and make for the best source of heros/monsters/war engine slots, but a classic error is taking too much of them.
A classic error of new players is taking too many hordes, resulting in several drawbacks :
1) in KoW number of units is usefull too, so taking too many expensive units like hordes might leave you outnumbered by your opponent who will then have a easier time flanking you
2) hordes have a very large footprint, so while they have a high nerve value, it's also easy to combo charge a horde with a lots of units compared to a simple troop or regiment, so if your horde is not correctly supported by other units, it might just present a big target to your opponent
3) the large footprint also mean that they are hard to manoeuver, so taking too much hordes might leave you with not a lot of way to deploy them in ways that they can support each others, and combined with the other problems this again increase the risk of being outmanoeuvred by your opponent and flanked.
Usually in most armies I will usually take one or two infantry hordes to form the core of my center, while the flanks will have smaller and/or faster units (usually a mix of both)
Note that a tough monster on a small 50x50 base like the giant might be a good support for those hordes, his relative small base will allow it to eitheir put itself in the middle of two hordes, or to support one to the side. The monster due to its small size will then be able to combine his own attacks to the horde or other nearby units depending on your needs.
The small footprint of such monsters (and some heroes) helps slightly lessent the problems with the large footprint of the hordes.
Of course there are exceptions and ways to make a horde army work (usually with cheap hordes like ratkin or goblins), when I play my ratkin for example I have 3 or more hordes and I usually use a different tactic where my army is deployed in multiple waves, for example a cheap slave horde to the front with a more expensive horde behing it, so that the slaves screen the more expensive unit and when they die clear the charge lanes.
Try imagining how you might deploy your army by putting your units (or cardboard cutout of the appropriate dimensions) on your table, and imagine moving them during your first turns even on an empty table without terrain. If you find that some units have their way blocked by other and don't contribute at all like you wanted because of that then you probably have too many hordes or other large units. Of course, if you have some units kept in reserve by design like in my ratkin example, then no problem, but it's usually better to first master the vanilla tactics before trying the more exotic ones.
