I have done that at times yes. The Quasit can be Invisible and getting 2 attacks a turn (one using your action, one using your bonus action), potentially making 2 enemies poisoned for follow up with Summon Undead. They are pretty survivable at high level with 25hps plus another 15 or so from Inspiring Leader and at will resistance to all damage as a reaction. Not saying they won't go down in combat, but enemies usually need to actively target them or try to down them.
The Pseudo Dragon has an advantage in that it does not use your action or bonus action at all to make its Poison Sting (it is not an attack), so they get to use an effective action with a good chance of knocking most enemies unconscious at will. The downside is it is usually once or twice a fight before they die due to their very low hit points and no invisibility.
Usually I play with either 1 familiar or primarily 1 familiar during a campaign for thematic reasons, but if you are willing to mix and match it is more effective - summon in a Bat when dealing with an invisible enemy, get an Imp when on that raid into a red dragon's lair, Sphinx of Wonder when dealing with an enemy Wizard etc.