I'm a BJJ purple belt who started taking a judo class twice a week. I thought my years of grappling would give me a head start.
I was wrong. First class, a 130lb brown belt ragdolled me with an uchi mata. I couldn't grip fight, my breakfalls were terrible, and my standup game that I thought was "decent" was exposed as garbage.
Judo is a completely different sport and the learning curve is steep. But I'm hooked. The throws feel incredible when you hit them and my standup for BJJ has improved already after just a month.
If you do BJJ and think you're good at stand-up — go try judo. Humble pie is healthy.