100% agree - no point having lot of water movement at the top but multiple 'dead spots' at the substrate level. This is especially a problem for 'tall tanks' where all the water flow is from outlets at the top of the tank.The key is that flow is not the objective in itself, it is all about distribution/circulation and surface agitation as mentioned previously.
Checking your flow at substrate level is pretty easy if you have an in-tank diffusor (another advantage of an in tank diffusor ...) just place it at each corner of your tank at the substrate level and look at the CO2 bubbles, do they rise up more or less vertically and only start moving 'sideways' nearer the surface? Or do the bubbles start moving 'sideways' near the substrate.
when you see the bubble movement, you may understand why I am such a big fan of mini usb pumps (100litres/hr only) at the substrate level....😅
Of course, if you have a huge amount of flow, the risk of 'dead spots' decreases substantially.