A Note on Detachments in 8th Edition:
At first glance, detachments are a no-brainer instant addition to any list. They don't cause panic (which is amazing), they allow use of the many detachment rules which can change the course of an entire battle, and they offer a degree of flexibility that many armies merely WISH they could muster. In 7th Edition, if you didn't take detachments, then you were robbing yourself of the strongest Empire ability. I am in no way going to diminish any of these factors. Detachments can be extremely powerful additions to your forces, and you should strongly consider adding them to your list every time you begin building your army.
Unfortunately, there are now many reasons NOT to take detachments. 8th Edition changed the dynamic of the way the game is played in such a way that detachments are no longer an automatic inclusion in many lists. In fact, I have found myself leaving detachments out of my lists more and more. Let's run down the pros and cons of each detachment type.
Handgun / Crossbow Detachment.
Ballistic Skill shooting has lost a lot of it's glimmer with the advent of giant blocks of infantry all over 8th Edition. Whittling enemy blocks is now the purview of Mortars, Rocket Batteries, and mages, and the mighty Handgun has been relegated to a more pedestrian task of picking off cavalry and other shooting units. They are almost never worth it. The only task I think they would be useful for is protecting warmachines, and then you would be better off with a non-detachment unit that can take a banner and musician.
Archer Detachment.
Just .... no. Bows are move and shoot, and skirmish is nice, but these guys should be 5 points now, so 8 points is just too much for a throw-away screener unit that mostly gets sacrificed in a role that Free Company could do better.
Combat Detachment (Swordmen, Halberdiers, Free Company).
They get counter charge and supporting charge, which are still powerful. But in order to disrupt ranks, they need to have two full ranks themselves. Disrupting ranks does not negate steadfast, so hopefully your parent unit will be able to do that on it's own. For the most part, Swordsmen are now best for this task, because they have the best survivability to help keep their ranks. Halberdiers are good too, because they will help with some extra kills, but they will also take more in return. Free Company, which once held the rank of Best Detachment choice, are now rarely seen in my lists, and then only in 5 man detachments used to draw out or redirect nastiness such as fanatics or frenzied troops.
Finally, there is the Big Question. Can I include detachments, and still meet my Core requirement? They don't count towards your Core choices! This was not a big deal in 7th Edition, because you could field three core units with full complements of detachments no problem. In the world of 8th Edition, bigger units, more artillery, and more characters means fewer points to go around. That 25% Core Requirement has become a premium. This is the number one reason I find myself leaving them at home. Their benefits do not outweigh the extra Core Units I have to take to make up for their presence in my lists. It is unfortunate that this is the case, but the fact of the matter is that two units of 40 halberdiers with full command, and a unit of 20 Crossbowmen just does not equal 25% Core, and adding detachments will only make it MORE difficult to meet that requirement, not to mention that maneuverability seriously begins to become a problem with such large blocks all over the field.
Detachments have their place, and they are a great tool that we have in our arsenal. Just don't feel that you HAVE to use them, and if they don't fit into your list, don't feel that you have to force them in there even if they weaken your overall battle plan.