I haven't played Halo 5 for a while now. TitanFall 2 MP pulled me away, and then I caught the bug for Rainbow Six Siege on PC.
So anything I say is from memory, I'm not sure what changes have been made since 2017 or 2018 or so.
The major problem with Halo 5 is lack of modes and lack of maps.
Warzone was a bad bet. Simple as that. If they had added Warzone on top of BTB and Arena, then I don't think there's a problem. The idea that Warzone was a substitute was stupid. Over time, they added BTB back and a lot of the best maps were community map. So they didn't look great, but they played great.
As far as Arena, I think the maps suffered because they were all too close in size and feel. A lot of them felt laid out specifically to make use of the new mobility features, which is fine, but if they all serve the same purpose they all start feeling interchangable. I think Halo 5 needed more small-medium and medium sized maps in Arena. I know 343 didn't want the Halo 5 meta to retreat into a defense-first game like Halo 3 ultimately did, but for the sake of sheer variety, there should have been more maps that allowed for long-range engagements and more defensive play. Halo 5 Arena was all small-maps, all-offense all the time.
As far as the sandbox... I think it was close to perfect.
I personally disagree that the Magnum is overpowered. It's a 5 shot kill weapon (4 body, 1 headshot ...or 4 body followed by 3 more body shots) with lower range than any other precision (ie, weapon capable of crit damage to the head) weapon in the game. The BR is a 4 shot kill (3 body, 1 head) from mid-range, the DMR is a 5 shot kill (4 body, 1 head) from long range. So basically you have the BR as the 'average'. You move into short range, the BR is still effective, but not as effective as the Magnum. You take the BR into long range, and the BR is still effective, but not as effective as the DMR. You have two range-specific specialty precision weapons and one more versatile mid-range weapon that you can use to fight off the other better options long enough to reset the engagement to your advantage, be it through retreat, grenades, etc.
I think the problem people have with the Magnum is that the bulk of the maps they play are smaller Arena maps that intentionally or not, favor the Magnum. At least at higher levels of play. The major issue with Halo 2 and Halo 3 PVP at higher levels was that everybody spawned with essentially useless weapons (SMG, AR) compared to the BR... so whatever team picked up the BR first and controlled them had a hilarious advantage. That's why competitive Halo in both games started players with BRs. Halo 5 starts players with a very viable weapon... the problem is the maps turn that viable weapon into arguably the best weapon by virtue of context. If 90% of engagements start at close-mid range due to map design, than the weapons that benefit from longer ranges never see their time in the sun in the base modes.
As far as grenades, honestly, I never had much problem with them.