One problem I have with it is that the classic Aliens can already spit acid as evidenced by A3 and A:R but that instead of giving them that ability, they created a whole class dedicated to it. Some would say it's important to have more than one class of Aliens, but a lot of Alien/AVP games/comics seem unable to introduce new varieties of Aliens without undermining the classic one.
As for the sacs, design-wise they're a bit goofy, but I do like how they may hinder it's ability to sneak around, potentially making it a little challenging to use during gameplay.
So, it's an alright adition. But I wouldn't have minded the classic Aliens also being able to spit acid, albeit a weaker one, with the spitter's being more much powerful, instead of making the former unable to do it at all without a valid explanation unrelated to gameplay mechanics.