Of Tamriel Rebuilt ? It is good, in line with the vanilla game in terms of pretty much everything and even better in some areas, particularly some exteriors (Boethian Mountains and Lan Orethan forest for instance). It is also huge. In terms of quests content, most people would recommend starting in Old Ebonheart and while I haven't really touched this part myself I'd agree, even if you do it with an already high-ish level character.
That said I don't think it is for everyone (myself included) but it is definitely a must-try mod.
As for level-scaling in Morrowind....it is there, but it's not as noticeable as in, say, Oblivion. You might have noticed though, that you tend to encounter more Dremora Lords with daedric weapons and Golden Saints now that at the start. But at the same time you still get low level creatures like scamps and clannfears so the level-scaling doesn't feel weird in the end.
Similarly, containers in smugglers' hideouts tend to have more grand soul gems, dwemer gear and scrolls of Ekash Lock Splitter around level 20 than they do at level 5.
We have to distinguish between level scaling and levelled lists. Level scaling (I would say it's generally agreed) is when the enemies in the world
scale to your level. This doesn't mean new enemies spawning, necessarily, but that NPCs for instance have their own health, skills, equipment etc. increase alongside the player. This is the cardinal sin of Oblivion, leading to the Bandits in Glass armour meme, as well as Ogres that take 40 hits to kill as you progress past Level 20. Level scaling is not present in Morrowind, since all of the NPC and creature stats are entirely static. Levelled
lists are present, and used extensively in Morrowind. They're also used even
more in Oblivion, and much more aggressively. A quick break down, just so we all understand what's going on and how it actually works in Morrowind, using Oblivion as a point of reference and comparison:
Levelled lists in Morrowind: Overworld and dungeon spawns can be static, such as the Daedroth, two Dremora Lords and Golden Saint in Ibar-Dad. Those enemies are there at Level 1 or Level 25. Alternatively, spawns can be a Levelled Creature, represented by a Ninja Monkey in the Construction Set. The in_dae_all lists tell us which Daedra are supposed to spawn inside interior cells using them (such as Ibar-Dad). At Level 1, only Scamps can spawn. At Level 7, Flame Atronachs and Clannfear now have the ability to spawn. Extra spawns are added until this particular list caps out at Level 16, when Golden Saints and Winged Twilights spawn. Each additional spawn has an equal chance to spawn, meaning that at Level 7, there's a 33% chance for each enemy to spawn. At Level 16, you're looking at 9% for each type of enemy. Even at Level 50, Golden Saints and Scamps still have an equal chance to spawn. Weapon and Armour levelled lists work similarly. A Dremora uses the random_excellent_melee_weapon list, which means even at Level 1, it will spawn with a Dwarven weapon. Once you hit Level 14, a few Ebony weapons show up, and then through 15, 16, 17 and finally 18, you have all of the different Ebony and Daedric weapons fill the list. So you have an equal chance to find a Dremora Lord with a Dwarven Claymore or a Daedric Claymore, even at Level 50. If it feels like they spawn with
a lot of high level gear, it's because the combined Ebony and Daedric weapons outnumber Dwarven weapons 2:1 on this list. All human-type NPCs have static gear, as far as I know. There are also lists like "random daedric weapon" that always spawn a Daedric weapon, even at Level 1.
Levelled lists in Oblivion: Probably 90% of the game is levelled lists. This includes NPCs in towns, so you'll find some of them running around in Ebony, Daedric and Glass at Level 20 which, tbh, is even goofier than the Bandits and Marauders. The number of enemies who actually have static gear is very short (Azani Blackheart and Umbra off the top of my head). The biggest difference, though, is actually that the vanilla levelled lists for both equipment and creatures, stop spawning something once it is 8 Levels below the player. This is why certain enemies entirely disappear from the game once the player becomes strong enough. A great example is the Boar, which starts spawning at Level 7 and will stop as soon as you hit Level 16, since it doesn't appear in any "nuisance" creature lists (like Mudcrabs, Rats, Wolves etc.) and doesn't have any static spawn points. Flame Atronachs suffer a similar fate. This is why, once the player hits Level 25+, they start to only see a few creatures from each enemy category and why mid tier equipment all but disappears from the world.
Levelled NPCs in Oblivion: This is where the true insanity lies. You get formulas for creature HP like 26*(lvl-3) which means that at Level 20, they'll have 442 HP, and at Level 30 they'll have 702 HP. Melee damage will be determined by a formula like 22*(lvl-3)/2. There's no cap on this and player damage and armour values
do eventually cap out. Morrowind doesn't have anything like this, although Tribunal and Bloodmoon have their fair share of degenerate health sponge enemies with HP stats in the thousands, when the strongest vanilla enemies cap out at like 350-400.
Aside from rambling about the levelled list/scaling pedantry, I think that Tamriel Rebuilt is well worth playing for anybody who enjoys Morrowind, but is in no way "essential" to enhance the experience. It's for people who enjoy the world enough to basically LARP. I don't mean that in a negative way, but rather that they enable you to ignore main quest content because you now have two or three times as much Mages Guild content as the base game, meaning that it can easily fill up 30 hours and basically comprise an entire playthrough where the player reaches Level 20+ on their own. I actually made the mistake of doing Main Quest content on my last playthrough, and ended up outpacing TR content, not realising just how much of it there was and how good it was after the Ashes of Empire updated all of that old Firewatch and Helnim stuff. I'd normally say that for a Morrowind playthrough to feel complete, you need to do a faction + MQ or two faction questlines. That's usually enough to give you enough gear and enough levels to feel satisfied with the playthrough, and arrive at the point where you're too strong for anything to challenge you, which is the real endpoint. Using the Mages Guild as an example, again, Vvardenfell houses 33 quests according to the UESP. Tamriel Rebuilt now has 34 quests for this faction. Combine it with the various miscellaneous quests that you inevitably end up getting sidetracked with, and those almost comprise a game in their own right.
Not every faction is currently up to this level. Redoran hasn't been touched, and the Telvanni stuff is outdated and doesn't represent the current design philosophy. Other factions like the Imperial Legion and Cult are not expanded quite as much, which is too bad since their vanilla questlines are bad (IL) or too short (IC), but the extra content is still welcome. In the case of the Imperial Legion, the quest design is also a massive upgrade over the rushed product present in vanilla. The actual world design is arguably better too (prop density and placement, etc.) but there are mods such as BCOM and OAAB which bring the vanilla Vvardenfell up to par in that regard, and just about every enthusiast will have these installed unless they're a true vanilla purist.
