Combinations are the most difficult set of mechanics that I've ever had to adapt to a tabletop RPG. Even after several iterations, each one simplifying them further, they still take a full page to describe and explain. There are only two Combination Upgrades, but they have very long entries, their own set of common unique rules and possibly the longest sidebar in the book explaining how they work in corner cases. It's complex stuff, but I think most people will agree that's kind of inevitable considering the subject matter.
I mean, there's so much you can do to simplify "All PCs are combined into one" in a game that is designed around having 3-5 separate PCs, each of them with lots and lots of unique abilities plus their own strengths and weaknesses keeping them balanced to their environment. It's not just difficult to adapt, it is also difficult to balance.
Let's start with their common rules: First, everyone has to take the same Combination Upgrade, that's simple and obvious enough. Second, all Units are treated as a singular Unit. The book at this part simply tells people to deploy preocombined for the sake of simplicity and leaves the rules for the Combination Action to their own sidebar - they are just that darn long. We'll go over those when looking at both of the proper Upgrades, and instead talk about Subpilots now.
Subpilots can only take Utility Actions, of which Maneuver is the most obviously powerful. Indeed, Maneuver is the one Action that pretty much justifies playing a Combiner by itself. The Utility/Offensive Action system was written with Subpilots in mind from the beginning, which might seem a bit weird since there's not that many Utility Actions a Subpilot would want to take, but that's why there's Upgrades that add more Utiltiy Actions to the repertoire. The first Subpilot Maneuvers and the second can use Boost or Disengage for positioning. Additional Subpilots need Support and Restoration Upgrades. The idea is to give Subpilots something to design their builds around other than just getting stats and maybe an Internal Upgrade or two.
The traditional sentai-esque combiner where one person shows off and the rest of the team just sit there and shout things wouldn't be very fun to roleplay. In BCG, dedicated Subpilots get to build around their role by taking Supports, Restorations or Upgrades that synergize with Maneuvering and make movement and positioning easier. It's powerful and fun, to the point that GMs should consider throwing one or two Super Combiners at the PCs to see how they handle it.
The downside is that Super Combination is very complex. It involves slapping a whole character sheet on top of someone else's, recalculating Attributes, and choosing one Area for the Subpilot to claim it as theirs. I simplified this and streamlined the Upgrade to the essence of combinations as much as I could and I doubt it could get much simpler than this without sacrificing what makes it cool. It is still complex. A Super Combiner that deploys precombined gets to have fun with a really powerful build without the complications of the Combination Action rules and can use a single character sheet, which is going to be crazy overstuffed, but at least it doesn't need multiple sheets.
Mid-Battle Super Combination is so complex that the rules for doing it are over a hundred words in length, and I think that alone does a good job of explaining the problems with it. There's a reason the book just tells you to deploy precombined. There is one problem with this though: Combining mid-Operation is a very tropey thing that mecha fiction does a lot of the time. Not all the time. But a lot of it. BCZ would later give Super Combiners the Invincible Super Combination Upgrade to fix this issue. In doing so though, it makes Super Combiners even more complex, because now you have to use the Combination Action rules. Though, to be fair, it makes them fifty-something words instead of over a hundred, so that's an improvement.
This is the other common kind of combining super robot (more in the vein of Getter, Arbegas or Aquarion) which is much friendlier to the idea of a balanced party where everyone gets to do cool things. Unison Combiners are generally weaker than Super Combiners, but have two extremely powerful abilities: 1) A single-shot super Synchro Attack that costs only one Action 2) A lot of utility value in being able to freely share Genre Points. The source material often makes a big deal about the pilots being in sync with each other, having three hearts beating as one, etc. I think those abilities do a good job representing that kind of teamwork.
They're still complex, but not as much as Super Combiners, because all Units are distinct from each other. You can do a Mid-Battle Unison Combination, but there is no real benefit to it and you should just always deploy in the Combined form. They don't have a powerup Upgrade like Invincible Super Combination, because at the time I thought they didn't need it, but I wonder now if maybe they should have had one. Feedback here would be appreciated.
Writing about Extra Areas, Alternate Forms and Combinations in a row is painful because they were the genre conventions most difficult to adapt to an RPG. Combinations weren't just the most difficult to adapt, they were also the undisputed champions of just how complicated you can make an ability this side of D&D 3E's Polymorph spell. Deploying precombined removes most of the hassle and means you just need to make a single sheet for the combined mech, so I'm definitely glad that I kept them this way for BCG before BCZ went and made people want to Combine Mid-Operation.
I wrote a lot about all the problems with Combinations, but I do like how they play once you've solved those. I think they nail what makes combiners cool in mecha fiction while improving the formula and giving Subpilots more to do. But enough about Combiners! Now that we're done with the Trinity of Messy Adaptations, we can move on to more positive subject matter!