I used to do a fair bit of mix.nahl's about a year ago. The pugs weren't terrible when they started and it was generally good practice.
My main issue with hl pugs, at least from my experience in mix, are that people only pick their friends or who is popular. Skill wouldn't get you picked. No matter how well you've played in the pugs, if you weren't already in the "in-crowd", you have to just hope you get picked, and probably only do end up getting picked 25% of the time.
Also, idk if this happens in 6s pugs, but its annoying when people add on their joke off-classes and then get picked over all the people who added on their main for that class, play terribly, and no one cares.
When I got on a team that time last year, I realized how much infinitely better it was to pugging.