With no UX skills on the team, there’s a good chance that whatever the team produces will have a poor UX. Probably close to a 100% Anything designs that aren’t a poor UX are just accidents that work out in the users’ favor.
UX is a complex field because there are lots of complexities: accessibility, mobile design, working in government agencies, working in an agile development process, and dealing with cross-cultural needs, to name a few. Yet, these are edge cases. 80% of UX work is quite routine and when learned by everyone on the team, creates consistently good designs.
The first UX skills anyone learns is to shift their thinking from the requirements of the system and the business to the experience of the user. They become sensitive to what it’s like to use the designs they are creating. It’s rare for that to result in worse designs than what you had before you developed that sensitivity.