Back in the (very) old days, Nader had an ammo count and they switched the grenade launcher as a cooldown ability because of the spamming capabilities she had. Once you had the grip on where to find the ammo, you could just spam nades endlessly. Cooldown abilities directly limit the number of shot per period of time, which is quite long here. In other words, regardless if you’re caring about your gl ammo counts or not, you’ll be able to fire a defined numbers of grenades. So it creates an interesting choice for the players : spam everything, create total havoc and then turn almost useless because you’ve depleted all your grenades (strong fight situations) or use them with care and keep a good punch if necessary.
For Rhino, the thing is a bit different because, unless you go total retard not caring about heating the minigun at full load, it is pretty much able to fire 24/7. Putting ammo on this would not sound as useful. Just for a simple comparison, TF2’s heavy has no overheating issues but rather small ammo available. Imo, that’d be very hard to handle both heating and ammo counts at the same time. Not to mention, good Rhinos (those who are pushing not camping some random secure positions) already have to manage their life pool quite strongly.