Enemy Features offer options that are, usually, more fit for Enemies for flavor reasons but not forbidden to PCs. Likewise, regular Features aren't forbidden to NPCs and they're often crucial to making some Grunt and Boss designs work. Grunts have very low MP Totals, especially early on, while Bosses don't have any normal Upgrades unless you sacrifice Boss Upgrades to grab them, which is probably a bad idea. Among Enemies, Flyer and Terrain Specialist are the most popular PC Features, but Extreme Fortification works great for some builds too. Without access to Features, Grunts would be slightly weaker but Bosses would lose too much flexibility and much weaker.
But enough about PC Features, let's have a look at the NPC ones:
Biological transforms extra Energy into Threshold, but at a very slow rate. Combat is usually too quick for it to amount to much of value so a lot of the time this is mostly a downside. Grunts don't have an use for this and would only take it for flavor reasons and Rivals have better options for self healing. For Bosses, this makes them actually give a crap about Maims, which is a pretty big downside... But there are some Bosses which have enough spare Energy and ways to buy time for themselves that this Feature makes a decent option for them. Two Capstones, Energy Drain and Hypersonic Striker, work particularly well to make Biological work the downside. What is more, This is a very flavorful drawback for Bosses that makes choosing Areas to Maim relevant. It can even be a nice way to compensate for Oldtype for GMs that feel bad when giving that to their Bosses. Overall, Biological is kind of bleh mechanically, though it has neat flavor.
Extra range won't compensate for lack of mobility unless the user is very well protected by Terrain. Thus, this is a Feature used for Operations designed around Enemies bunkering up with PCs who often have to cross various kinds of harmful Terrain in order to reach them. Fortress Grunts are immobile turrets that emphasise their glass cannon-ness being extra vulnerable. Rivals don't really have much use for this outside of Guardian of Steel gimmicks where the whole of the team can't be moved. Bosses lose out on some great defensive abilities based on Movement and their aura-type effects are a lot less useful... But the extra Range makes many Weapons, particularly the Bursts, extremely brutal. Useful and flavorful, I like.
Invasive Enemies are a cheap trick. PCs without an investment in Systems are reduced to 10 Threshold total while PCs with an investment in Systems can have up to 20 or 30 depending on their luck. It's nasty stuff, to the point BCZ includes a new Genre Power just to counter them, because they're not just powerful - they're random. Invasive Enemies work best if they happen, say, once every 4-5 Operations or every 2-3 if there's one or two PCs with the Hang in There! Power. Grunts will often fail to do enough Damage for the Invasive trigger, because their strength is in making many weak attacks instead of a few strong ones. Rivals have enough attack power and abilities that deal automatic Damage to make it interesting. Bosses are like Rivals but they also have Crush the Insect to push PCs into the fourth Layer that automatically defeats them, making them the best and most obvious Invasive build. All that being said, Invasive Enemies can be very frustrating and unfun, so I'll give them a passing grade but just barely and add 'use with care' to their report card.
Squadron is my favorite Enemy Feature. It is how BCG represents swarm-type Enemies, a very important job for any heroic action RPG, and pulls it off in just two short sentences. In terms of optimization, Grunts use this as a cheap Antimaim that emphasises their natural weakness to area-of-effect Weapons. Rivals can use this to represent the ability to split into separate components and can be combined with anti-AoE abilities for a fairly optimized Enemy. Bosses gain the least from Squadron, since they're already mostly immune to Maims, unless they use this to compensate for the upside of Biological or using regular Upgrades and Weapons... In which case you have a really interesting Enemy in your hands. It is the Feature that does the most with the least text and a home run in my book.
So, in conclusion, I think Enemy Features are a pretty good set of abilities. Biological or Invasive need a bit of tweaking in the mechanics area, but all of them do good job of conveying the flavor of fighting enemies that are fundamentally different from the PCs. Squadron, in particular, is the kind of thing that could be the basis of a new NPC tier, for waves of Enemies weaker than Grunts.
Next: Boss Traits