Here's my quick 2 cents:
1 - Some people just don't seem to get along with the mono rail. I personally have never had any issues and don't find it drastically different from a twin rail. I do think it's less predictable on steep side hills when compared to the twin rail and it does have some tendency to climb up or wash if it's steep, but I actually prefer it overall for the majority of my riding. With that said I also think that throttle control and rider position play a big part in that and I rarely have issues unless it's really steep.
Having put around 200 miles on the Catalyst so far I feel they have built in some " rigidity" to the new chassis and so far I think it feels more stable and predictable when sidehilling. The catalyst chassis is definitely a big improvement over the ascender and that is with the basic shocks that came on it which in my opinion are way too soft and don't have any adjustments besides spring pre load. I would describe it feeling slightly more like a Polaris Matryx but it moves side to side with less effort. I also noticed how compact and small it feels compared to my Alpha and the Matryx. Bonus points for the attach system so far. Not sure about overall longevity/durability but a nice feature for sure. With an 858 in the Catalyst chassis, it's going to be a solid sled.
2 - Fuel really depends on your style of riding, terrain, amount of trail, etc. I rarely pack fuel unless we have a long trail to get to the riding zone. Based on my riding group and generally what I have seen (most all newer sleds Matrxy, Gen 5's, 2023 Alphas, Catalyst), Ski Doo seems slightly more fuel efficient than Polaris and Cat but it's really hard to say unless everyone is riding the exact same mileage, time, and so on.