IMO the tough enemies aren't rare enough. They easily flood all the areas, making getting through some areas very painful and slow. I really dread unlocking more of them.
And I definitely agree that alien speeders should be minibosses.
There's something that needs to be accounted for here. The difficulty you play with is a factor, but even more so, the input method you use determines which things you consider to be threats. If you play with auto-targeting, you have time to focus on dodging all the homing missiles. This is not so true if you're playing with the mouse -- in this case you have to focus on finding the enemy at the same time as dodging, and dodging homing enemies (like the speeder's attacks) becomes quite challenging. If you're playing with the keyboard or gamepad, the game is even more challenging, since you're never guaranteed a hit against an enemy -- you have to change your position to align your shots, and you cannot fire backwards. While it's easier to fire off a whole bunch of spells (vs someone playing with a 2 button mouse), the rest of the fighting is far more challenging (and I mostly like it that way). The upshot of this is that every input method creates a different game experience. This is why I've long insisted on trying to standardize an input method most people can deal with.
In any case, don't be surprised if someone posts that the enemy you find easy to deal with is annoying to them. Bats, for example, are extremely tough to deal with effectively with the keyboard, since you can't just hit them at whatever angle they happen to be from you. The best way to deal with bats is with something like ice cross, and even that requires some aiming (or at least timing). Even desert burrowers are a challenge with the keyboard, since you can't target the ground easily. This means you want to snipe them while jumping or use spells that fly with an arc like Throw Rock to kill them.
I'd therefore appreciate it if people could state their input method (auto-targetting, mouse, or keyboard/gamepad) as well as which enemies annoy them.