At very low levels that second full hit die for the Ranger makes them much more survivable
It's always necessary to look at things in terms of XP and likely ability scores. The 1st level ranger is clearly very strong, but they only gain a +3.5 hp bonus on the fighter. Plus, the Ranger needs somehow to find Intelligence and Wisdom to go with its required Strength, Dexterity, and Constitution. You're really unlikely to have 16+ in five stats, so somewhere the Ranger is sacrificing. Meanwhile, a competent fighter only needs Strength and Constitution. And really, just 18 Strength is enough, the 17+ Constitution is gravy, and if you need to you can go Dwarf to bump that (before level 9, Dwarf Fighter is amazing). There isn't really anything you can do to make Ranger work out so easily. So, you have to deal with the fact the fighter gets a 10% XP bump and it's really likely that either the Ranger doesn't have 16 INT, 16 WIS, and 16 DEX (and so doesn't get a 10% XP bump) or else doesn't have 18 STR or 17+ constitution. So that extra HD only lasts only like 1800 XP, at which point you are now behind on hit points until 2250 XP. And then you are behind again at 3600 XP to 4500 XP. So you aren't that much more survivable even if you are lucky enough to be matching the fighter in Constitution while rocking universally good stats.
And again, not having henchmen or men-at-arms' servants at low levels is tough, and that missing weapon proficiency never hurt until specialization came along and ate 3 weapon proficiency slots for your first weapon. You don't contribute quite as often until level 3 and you get your 4th slot, and then at level 6 the fighter gets his first bump with his non-specialized weapons and you don't until level 7. It's a good solid class but it's always struck me as that class you go into when you don't have a lot of 17's and 18's but you don't want to suck.
Your perception of it therefore might be based on very restrictive ability score rolling (hence no Paladins) because it gets a lot of bonuses that aren't centered on ability scores and yet is also a bit hard to qualify for, so when you do see one they are a bit above average.
Well, all I can say is that our long-term stats show MUs have roughly the same survival rates as everyone else. Worth noting, though, that we long ago added in the concept of "body points" - for PCs this represents 2-5 additional hit points at 1st level with that number locked in for life once rolled - and I think that makes a big difference (though my games are still bloody lethal for all classes at low level!).
Again, this is part and parcel of just how little I can say about your game because every single conversation with you has always revealed several house rules in every post.
And again, a lot of this can be GMing style and encounter design. My experience with D&D as a GM is that no matter how skilled the party, there is always some encounter - a trap, an area attack, a burrowing monster, unlikely surprise, mass missile fire - complex enough that it disrupts the normal meat shield strategy. For every other class, that produces a "oh heck" moment but then within a round or two the situation can be righted, and the cleric can sustain and recover. For M-Us, this produces death, rather instantly regardless of the character level. It's not just the 1d4 HD at 1st level, but also the fact that the burst damage of monsters of equivalent level to you just always keeps up with what an M-U is expected to have. I just kill M-U's left and right, and have done so for 40 years. The fireball from the enemy mage out scales none of the party, but does tend to out scale the M-U. The giant tossing boulders hurts, but is lethal if any amount of focus is placed on the M-U. The dragon's breath, the trap that gets missed, the umber hulk at close range, the second griffin of the mated pair, whatever it is, the M-U goes down.