Sounds like hill sprints can serve you really well given your experience with them. They serve predominantly as a neuromuscular exercise, in a couple fashions. First, as a technique reinforcement: both the terrain (steep) and effort (maximal) demand that you be quite dynamic with your movement, and that tends to put you into a forward leaning, powerful position which supports good running form. Using your posterior chain muscles to propel the movement by driving through with your stride is another positive aspect of these.
Second, these serve in a similar manner to max strength in the gym – the load is very high but short-lived, so you don’t develop much fatigue via glycolytic pathways. It’s not unlikely that you sense of them being “easy” stems from not having those bundles of motor units recruited and trained for the output required – hence, the more you can do these, the better!
Even with some snow on the ground these can still go: pack out your hill for a few laps as your warmup (low intensity) to create better footing, and use Alison’s strategy of screwing 1/4″ sheet metal screws into the tread of your shoes to improve traction. Alternatively, you can use stairs indoors; while not as specific for running form, they still initiate the muscle recruitment and high tempo, both of which are preferable for this activity over a jumping workout in the gym, which tends toward muscular endurance.
Hope that helps,