I ran the campaign on normal and the limited amount of ammo I could carry for all guns was really distracting and detracting from the experience.
I know this has been the trend since Halo 2 but I was really getting annoyed since the available weapons in some areas were only tickling the enemies specifically the Warden Eternal and Knights. I am all for adopting to the situation but only if there is an effective way to do so.
I noticed this in halo 4 solo legendary run and halo 2 solo legendary run. There’s a lot of RNG based missions or areas too. Sometimes you just get the shhit stick when it comes to enemy spawns and reactions. There’s always a way through though. I know you’re mentioning ammo, but a lot of it also has to do with conservation and being in the right place at the right time.
I wish you could carry more 
I found myself picking up 20-30 weapons per mission, the lack of reserve ammo is pitiful.