I've been wracking my brain on how to implement all the things I want to include in what is essentially The Knights of the Round Table, the Giant Stompy Robots They Pilot, and the Nasty Monsters They Slay: the Tabletop RPG.
The game doesn't need, but I most certainly want, the Traits and Passions subsystems from KAP. Sure I could use the Motivations subsystem from BRP Mecha, but I prefer the brievity of Traits and Passions as well as their commonality across characters. In my last K&K campaign, sadly years ago, the players and their PKs all acted in suitable anime manners using the basic shell of Pendragon.
Giant stompy robots should have hit locations which should lead to limbs blowing off like all good giant stompy robot animes, and battle tech, have demonstrated as being good. (Un)Fortunately, almost all mecha rpgs feature this to an extent, and all the ones I've directed to examine do. Of the four games, BRP Mecha, Mekton Zeta, SilCore, and BattleTech, I've been looking at integrating in either direction, only SilCore is the least obvious but might be the most interesting regarding the lost of mecha limbs and similar concerns.
I want the mechanics to be as simple and consistent as possible across the two main mode of campaign play, the courtly human-scale and the stomping around in stompy robots-scale. I want the primary resolution mechanic to be the same. While there might be value in having the way a human-scale combat resolves vs a mecha-scale one by dint of them being conceptually as well as mechanically different, I would rather reduce the inevitable confusion that will arise from switching mechanics. I've a large amount of time just reminding new players which die to roll to deal damage in D&D.
This leads to a problem with using SilCore because damage is determined by the amount an attack role exceeds the defense and is then multiplied by the Damage Multiplier. I really like the way in SilCore constructs and resolves mecha combat, but it requires using its whole resolution and possibly stats systems to keep from ridiculous one-hit destruction that could easily result from using KAP's roll-under w/ a d20. There is also the slightly cute way SilCore works, the die pool plus modifiers but you only keep the highest roll adding modifiers, which is conceptually complicated when compared to KAP's "roll this die and you want to get the highest number without going over your skill/trait/passion; if you get exactly the number it's a critical hit."
Mekton Zeta would be less complicated, as mecha-scale damage is fixed, and I could just work out a different way to detail how the mecha affects the skills of the pilot. Similarly, BattleTech is fixed damage and would require some modicum of conversion. Naturally, the simplest thing to do is just do what I have already done before, use BRP Mecha and continue to ignore the swathes of rules I don't need.
The problem with using BRP Mecha, and to a lesser extent SilCore, is that BRP Mecha isn't intended to allow players to design their own mecha. The onus is on the GM, and the intention is to be emulating extant mecha franchise mecha in your game. It has no "build a mecha from scratch" section; instead, it has a "convert a mecha from your favorite anime" section. SilCore, Mekton, and BattleTech, on top of having loads of ready to go mecha, also has "build a cool robot" built into the game. Of course, I could ignore my desire for the players to have custom mecha, it's not like PKs in KAP can customize their horses, and decide what I want a rouncy, charger, sumpter, and sundry other mecha to look like. This was what I was doing a couple of years ago, but got frustrated with the lack of difference between derived stats with even large differences of base stats. That was because I wanted to stick as closely to how the book was written and the intentions behind the rules as I understood them. This an exercise in futility. BRP mecha is not well organized, seems skewed towards certain mecha anime genres, super robot, and has a large amount of cruft in the rules.
The only reason to not use BRP Mecha is that it has very few pre-built mecha in it, but that is a problem I solved a few years ago with a spreadsheet that automates the stats portion of the mecha building. I have a folder full of mecha cards that I did as practice, so it would be a relatively easy thing to get basic mecha to use for a Knights and Kaiju game.
The reasons to use something that isn't BRP Mecha are that they are all much older games, that are ironically still in print, unlike BRP Mecha, and there are communities available to ask for advice. BRP Mecha's short life-span along with total lack of other supporting products mean I have to work without a net, which is so unnerving and part of why I stopped doing on any campaign work for it.
I can, as is my right, take those things from other games I want and use them to help me craft my personal mecha system. To wit, I like fixed weapon damage on the mecha-scale, I like the slow degradation of armor by attacks that penetrate it like in Mekton Zeta, and I like the heat management aspect of BattleTech, all of which should bolt into BRP Mecha, which in turn should bolt easily into KAP. I just have a problem with things like the cost of upkeep and the like, or rather the lack of them in BRP Mecha. I'll probably steal pricing from BattleTech.