To answer your question Daniel, it really depends on what the team wants and what they are looking for as well as how much resources does that specific team has.
Now let’s look at a veteran team who has members who have been through this design process a few times, engineers to correct their mistakes, resources to make what they want. Ofcourse they will want to go with a custom frame where they have the flexibility to work with what they want. After given the game, they will design chassis such ways to satisfy their needs.
Let’s look at a rookie team, or a 1st or 2nd year team. They have dedicated members, engineers who are there to help. But they haven’t been exposed to FIRST or they haven’t been to as many competitions as a veteran team has. What I am trying to get to here is experience. Due to less resources, teams may want to go with the kit of part chassis.
In both ways, there are pros and cons. You ask how? If you use a custom frame, it will take time to design it, machine it, put it all together (like Cody pointed out in his post). On the other hand, if you decide to use the kit of part chassis, it will only take you the first week of build season to put it all together. That’s just one example.
There have been many teams that have been successful with the kit of part chassis. For example, Team 121 and Team 1625. The reason I used those two teams as an example is because one of them is a veteral team and the other is a 2nd year team.
My team (team 1345) is a 4th year team. In the first two years, they have used extrusion and aluminum tubing to make their frame. Last year, they used the kit of part chassis. This coming year, we will be using a custom frame, we will see how it turns out.
Bottom line is, if you have the resources to make a custom frame, then why not? But if you don’t have the resources, don’t take the risk.