These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
YMMV: Windom XP
Canon Fodder: The MS-98 and Milleniums in the Team Battle and Blitz Tactics modes.
The KD-07 becomes one once you realize that his Anti-Beam Reflection can not only reflect beams, but also recharges your bars and can withstand even bullets. Now imagine that… With a wide range Beam Spam attack, a Trap Beam, 10 funnels and some of the most powerful damage-rates of the game.
Also by deleting the Hit.x files from the mech, basically rendering it indestructible.
Some of the Game Mod qualify as that as well. Some are actually adapted to a different level of gameplay and the incompatibility between them results in unfair challenge.
Genius Bonus: Four of the KDs' names are less obvious and require some research to get their signification. The KD that's now known as "Vista" used to be called "Longhorn", the original codename for the OS. "Blackcomb" and "Vienna" were codenames for Windows 7, and "Talisker" in the Power-Up Kit was the codename for Windows CE 4.
Most Annoying Sound: A.I. pilots will seemingly spam their responses. In team battles, you can rarely even see or hear what you're doing sometimes.
That One Attack: KD-07's Beam Spam and freezing beams, KD-07-2's acrobatic jump-and-shoot, KD-03's Grenades in close distance.
That One Level: The two last levels in the Arcade Mode since the v2.000 expansion update. Fight an army of 4-6 Linuxes alongside a weak ally and win. You'll be thankful that the Amecha forums made trainers to even the odds for you.
The old final fight with the KD-07 & KD-07-2 as well.
They Changed It, Now It Sucks: Bootfighter Windom XP SP-2 v1.100 and higher has halved all the available mechs when older versions had them playable from the get-go. To unlock more units, you have to get at least ten kills in Survival Mode with each and every unit available.