Because the Oblivion rendition of Gamebryo is sheiße.

I've spent a few severals of dozens of hours trying to make Nehrim stop acting up, and this guide contains most of my observational knowledge. Some of it may be conjecture, as I don't have the patience required to extensively test every little thing, so take it with a grain of salt.
Unless stated otherwise, described behaviours were observed (read: tested) on two separate machines - machine A (Win10 1809, AMD, HDD) and machine B (Win10 21H2, Intel+Nvidia, SSD).

I'm not sure if it'll affect stability directly, but it should make Havok less likely to crash at high framerates. Seems stable enough, although somehow it doesn't like low CPU clocks very much.

Actually seems to be doing work. I have been using MessageLogger[www.nexusmods.com] to monitor events leading up to a crash, and lately it's been shutting down every hour with a message that read: After a few attempts at juggling mods, installing MoreHeap[www.nexusmods.com] seems to have given Nehrim a fighting chance, as at the time of this writing the first timestemp in my MessageLog.txt is at 2023/07/17 09:25:53 and the last is at 2023/07/17 14:27:10, meaning the game has been running for about 5 hours with constant alt-tabbing without crashing.

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Believe it or not, but these two innocent looking OBSE plugins cause most of the crashes you get in an out of the box Nehrim installation (specifically of the Steam version). I'm not sure why they're included, but disabling them prevents the game from ever crashing within the first hour.

tl;dr Disable these. Try Trifle[www.nexusmods.com] and Enhanced Camera[www.nexusmods.com] instead, though be aware that these are merely suggestions, which I've yet to test extensively.

Nehrim uses a version of Blockhead that's older than the one on Nexus, which "fixes persistent CTDs". I'm not sure if those happen in Nehrim, but updating doesn't seem to break anything.

tl;dr Try updating. If you're unsure about this, just leave the default one as is and get your game in line with the rest of the guide. If you're still experiencing random crashes around NPCs and Cell transitions, this could be the cause.

"Confirmed" to be unstable. Don't use it.

No issues. Consider using OneTweak[www.nexusmods.com] or Oblivion Display Tweaks instead if you want to play in Borderless Windowed.

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This makes the game stop being able to clear its memory blocks, resulting in hourly (at best) crashes.

tl;dr Do not apply this to your executable. If you have, revert to the backup or Validate Integrity of Game Files to restore the unpatched executable.

Makes the game randomly freeze. Permanently. This could happen even in the main menu, immediately after pressing Continue! (Only observed on machine A, which previously braved many games with DXVK, with great results)

tl;dr Volatile. You should do just fine on DX9, unless you're playing through Proton/Wine.

Breaks a lot of scripts, makes some quest stages crash the game.

tl;dr Do not install.

This makes the game crash every half an hour. Nehrim really hates it. (Only observed on machine A)

tl;dr Do not install.

Honestly, this thing does so much I can't say for sure if it caused problems on its own... it did help me hound down other wrongdoers, though.
Hooking CS, Hashtables and replacing the heap with TH3 improved performance significantly, but I had to neck it in order to minimize interference while probing for bad actors, and haven't gotten around to testing it uninterrupted. (Only observed on machine A)

tl;dr Probably unnecessary. If you're used to relying on it for performance, it might be time to start biting your fingers and deal with poor framerates, unless it doesn't cause any issues.

Somehow causes a lot of issues, and manages to corrupt the memory blocks. Sometimes causes the inventory to freeze the framebuffer, so you're stuck staring at a single frame while the game runs (you can even control it!) in the background. (Only observed on machine A)

tl;dr Seemingly unsalvageable.

While theoretically it should be OK to use, the Nehrim Launcher really dislikes when you mess with the order of entries within the INI, which, in turn, results in your INi file getting filled with blank entries. I'm not entirely sure how duplicate INI entries affect the game, let alone empty ones, but it can't be anything good - especially when Nehrim expects several of those entries to be specific things considered to be "invalid" by BethINI.

tl;dr Don't let BethINI touch the INI files unless you're prepared to face the consequences of your hubris.

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Oblivion is known[en.uesp.net] to eat dirt because of Codec incompatibilites. On a fresh Windows installation, this shouldn't bother you, but if you've been using it for a while there's a non-zero chance some ungoodware crept its way onto your PC.

If you get really desperate and want to try this, you have four choices:
1) Format your drive and reinstall Windows (The Nuclear Option)
2) Learn to operate regedit and correct DirectShow entries to their initial state
3) Find DirectShow Filter Manager on a website that isn't malicious (Good Luck! Even the link on this[en.uesp.net] UESP page is flagged.)
4) Install the K-Lite Codec Pack (There's adware in the installer. The codecs are fine, but you'll have to untick it when prompted.)