Classic slots are good for maintaining a steady balance but they will not give you big wins. Keep in mind they work on RTP too. So after a period of time you will see a patch of low paying hits and eventually you will watch your balance go down to zero. Microgaming and Playtech are notorious for this.
In my experience, classic slots can be better for bankroll management, mainly because the gameplay is simpler and volatility is often lower. You don’t get hit with as many long dry spells or huge swings like with some modern feature heavy slots.
That said, it still depends on bet size and pacing even fruit slots can drain a bankroll fast if you’re not careful 😅.