Regarding destructible cars, gotta say that I love those and always want it ON. Sometimes you get killed "unfairly" (e.g. get a good start and be at 1st, get no shield for 5 pickups and get 1 assassin and 2 scorpions in the butt), but generally works OK. Maybe you could nerf the weapons in terms of damage a bit, or give all cars a little more health? Or do a repair powerup which you get only if less than e.g. 40% of health and it repairs maybe 25%) Also when more powerups implemented, this can be balanced (you dont get missiles all the time when there are more powerups, or make them more rare).
BALANCING is everything ofc. You might want to think ahead the weapon balancing according to laps and player count (with more ppl you get destroyed more easily especially if you are first) so that they are most balanced e.g. with 8 racers and 5 laps. Also assassins and EMPs maybe shouldnt appear in the first lap, unless maybe for last player if (s)he is over 2000m behind. These kinds of things.
Speedbowl is a little boring for classic race, but fits perfectly for ultimate race. You gotta always think if you want to try and get away and gamble getting shields or stick with the pack maybe 2nd place and try to get missiles and gattlers etc this kinda things.
The arcs in Yuri are good IMO, gotta use boost to get over 'em or just let go of gas for a bit to not hit em when coming from the high tunnel. The tunnel enterance following that part is frustrating, but consistently doable with skill. Now, I kinda agree with you but on the other hand I want the tracks to be challenging. I like that I have to actually focus to get a perfect lap, and I like that I have to let go of throttle or even brake if I want to make sure I get the cornering /enterance/whatever right. And from these elements that bring you to full stop comes the balnce between Cyclon and Dreadnaught. I don't want the tracks so that you can go full throttle the whole thing. I like FIC outpost very, very, much too but it is in the edge of being too easy, due to the fact that there are only a few parts where you briefly have to let go of gas and probably 1 part where I use brake (in reverse mode last curves before the road splits and finish line follows). Increasing the height of tunnels I to reduce the collision I totally agree. Transport is fast, too (only couple of spots where I briefly let go of gas and 1 spot where I sometimes brake), but the obstacles are the great thing that make it interesting, so there I disagree with you. Transport reversed jump right after start I like, I like these kinds of things where you can totally f*ck someone with the right timing with powerups (MJAH MJAH MJAH!)
What you say about pickups, I generally agree. My solution to this would be that the pickups could be little less on the track and would often times require you to not take an optimal path, thus making you to choose between pickup and optimal line. That way you would also appreciate the powerup more, and maybe even save it instead of spamming it immediately in hopes to get a better one for just 2 seconds after. Also, the balancing thing. Assassins and EMps could be way more rare (increasing in chance the more further you are from the pack), but as said, this probably will be better once more powerups are implemented.