I don’t know about anyone else, but I foresee some pretty big issues arising from game mode specific challenges, assuming playlists are as basic at launch as they were during the flights. Picture this; you just signed in to do your challenges for the week, and one of them is to complete 3 stronghold games. No wins or anything, so you just hang onto it. You then jump into arena matchmaking, and it’s slayer. That’s fine, you’ve got other challenges to do as well, so you go for it. But you keep doing this, and not seeing strongholds. Do you reset the challenge?
More likely, I see people just quitting matches until they get the game mode they want. I know I did this. A few times during the flights because I wanted to get those weekly challenges done for the coatings. I know I wasn’t alone in this either. I can also see this problem being especially prevalent closer to weekly challenge resets.
I think there could be a few potential fixes for this issue, like getting rid of game mode specific challenges, or pulling them back a bit to allow players to complete them with any game mode in a given playlist. The simplest fix could also just be to let us pick game modes specifically.
Overall, I think the current system too heavily promotes either the purchase of challenge resets, or quitting matches to try and find the game type you need more quickly. But let me know your thoughts, whether you think one of these changes would be best, or suggest a change yourself.
Ah but consider this: Completing stronghold matches will usually not be your only challenge. The Flight had alot less challenges than what we’d normally be getting, so no matter what you’ll usually make some form of progress in getting xp in the form of a daily challenge which is typically like, idk, kill 5 enemies with an AR.
The only time I can see this arising is for a Capstone challenge at the end of the week, and then yeah I can see where it could be annoying if you can’t get into the proper matches. But anyone who is impatient and decides to leave the match instead will be properly punished with a ban 
I feel that since the Battle Pass doesn’t expire and that those challenges are weeklies and don’t need to be done in 1 play session that its ok. I also imagine that they will have a system in place at launch to prevent unobtainable weekly challenges for that week. If not, that might be an issue. I kind of like that I’m not being rushed to get weeklies done ASAP so I can get my pass done before the next one comes. But I see where you are coming from. Perhaps the idea you had of playlist challenges instead of game mode specific. Play 5 Matches of Arena - Play 2 Matches of Big Team Battle - Play 5 Matches of Action Sack. I can see that being more user friendly.
I can see where you’re both coming from, but for me the issue just comes back to player choice. Obviously on release, bans will keep the leaving side at a minimum. Overall, however, I still think it restricts player choice, especially compared to older progression systems that didn’t restrict xp to challenges only.