Asimov said there were three kinds of SF story. The action adventure is where the gadget is a means to adventure: a time machine, starship, giant robot etc. In terms of aliens we are definitely talking forehead aliens. Humans would serve just as well but we want to make sure people remember it's SF. These aliens are sometimes given a special power or ability to make them harder to fight but it isn't necessary.
The invention story is all about getting the parts and materials you need for the gadget and building it. This sort of alien can communicate with humans and might even appear very human but they have their differences. Part of the story involves working through these differences or passing some hurdles then benefitting from the aliens' special abilities. The Vulcans from Star Trek fall into this category. Their unemotional exterior, copper based physiology and mating patterns all provide problems that need to be worked around but they have telepathy and extremely powerful intellects.
Finally we have the gadget that causes social change. Aliens that cause huge social change make lousy PCs. Folks tend to worship them as gods.
A good campaign can use all three and indeed many series and works have al three kinds. You need some forehead aliens to carry stuff or fight. You need some of the ones a little off by humans standards to create an exotic atmosphere and complicate life and sometimes your adventure just calls for meeting with a Big Glowing Head (the smug bastards).
The second type of alien is my favorite. Even if there are only a rare percentage of beings humans could relate to we'd probably try to hang out (if we aren't trying to kill each other). They encourage roleplay (hopefully) and have more of a SF feel than say a Fiji islander painted purple with horns. Seriously if you need background characters have a human diaspora. Aliens should be special.
One of the things I've tried to avoid in my own writing is humanizing my aliens over time. Mukh for example is a gender swapping amphibian whose race has no concept of gender roles, courtship or romance. In fact it is rare for both parents to be present when an egg mass is fertilized. No one is going to convince him that his species is missing out. They're happy to spend that extra energy on other pursuits. At the same time they certainly understand love and affection as they are social creatures. Young are brought in from the wild when they achieve some sentience and their lungs develop and are raised by various clans. It works as well as human families or tribes.
Tivk is from a more humanlike race. His people prize rationality. In my take, their emotions are not repressed so much as nearly absent. When they do show emotion it is by subtle clues like body signs, ear twitches, small ticks that most beings miss. The entire species is psionic to some degree so they can broadcast what they feel to others of their kind. There's no reason for smiling or laughing or screaming.
They are long lived and as people change over time they realize relationships change. They have no concept of marriage for life. When two people grow apart they leave because it is the rational thing to do. If a relationship doesn't work out they will often terminate it or 'return' to an earlier point. Yes they can be lovers and go back to being friends, something few humans do.
Nok's people are just creepy by most human standards. They are given to long silences, and periods of quiet reflection followed by bursts of activity or violence. They are descended from predators that attack by ambush. While a human will attack a problem or obstacle until winning or exhausted Nok's people prefer to observe, find a weak spot and exploit it swiftly. Powers of observation are very
important to them perhaps more so than intelligence. It's good to see something coming after all and if you see something coming that you can't understand you can always bring your problem to smart person.
So sometimes they will just act in ways that if understandable are weird. Mukh likes eating small furry animals alive. He just does. Nok likes to pick a dark place for a nap like under a bunk or desk. Tivk can be clueless about human reactions or invite anger by not responding to them. they aren't going to 'get better' or apologize for what they do.
My human characters are quite as annoying to the aliens as vice versa. But they are only human.