If the VVI, FPM, airspeed indicator somehow failed you can use fuel flow to help you with your attitude (climbing or descending)?
Without an Attitude Indicator, changes in altitude at a constant power setting gives you a fairly good indicator of pitch. If your Airspeed Indicator, Vertical Speed Indicator, and Attitude Indicator all failed at the same time, you would use your Directional Gyro and your Altimeter to stay straight and level. You would cross check that with your Magnetic Compass and your Turn Coordinator.
If your Altimeter fails you, you would keep wings level with your DG, TC, and Mag Comp while monitoring your Tachometer RPMs on a fixed prop or your Manifold Pressure on a constant-speed prop. This would give you a more immediate indication of pitch than your fuel flow counter. The DG, TC, and Mag Comp in most aircraft work off of separate systems from each other to avoid all three going inoperative at the same time. They are your instruments to indicate bank and turn.
The ASI, AI, VSI, and Altimeter are all pitot static instruments. And iced over pitot tube and static port can cause them to be inoperable. Using you alternate static port or breaking the glass face of your VSI will make your Altimeter work, making this scenario a non-issue.
Using a built-in GPS or an external AHRS like the Stratus will give you an unofficial altitude in a pinch.