The loss of the multiplier changes things a lot, wouldn't you say? Not against single opponents in the lower levels indeed, as you noticed, but single opponents are few and far between.
Hrm, yes and no.
The loss of the multiplier increases a fight against 2 monsters by 50% and against 3 to 6 monsters by 100%, against 7 to 10 monsters by 250%.
But at the same time, a 200xp cr 1 creature is (according to 5.5e) equal to 4 cr 1/4 (50xp) creatures. So the ressource draining should be similiar.
I would say, that 5e just undervalued multiple monsters (like, one of them most common fixes was to ignore the multiplier).
At the same time ... I would have to build some encounters to just see that.
Okay, let's give it a go.
Level 1 Party
Bandits are 25xp each. So 4 bandits should be equal to 1 gnoll warrior (100xp).
Bandits
AC 12 --> Hitchance against +5 = 70%
HP 44 (4x11) --> adjusted HP = 62 / 15,7 per Bandit (accounting for Hitchance)
Attack +3, 5 damage x4 --> Hitchance against AC 15 = 45% --> adjusted damage 9.
Gnoll warrior
AC 15 --> Hitchance against +5 = 55%.
HP 27 --> adjusted HP= 49
Attack +4, 5 damage --> Hitchance against AC 15 = 50% --> adjusted damage 2,5
Special: Rampage once (one additional attack)
Okay, at first glance it looks like 4 bandits are harder than 1 gnoll warrior.
Now let's look at the actual fight.
Our party are 2 fighters and 2 wizards at level 1 (thats where I have the math for ready to go

).
On average a Character will do 10 damage per round of adjusted HP (accounting for hitchance).
So on average 3 bandits will be killed in round one, the last one in round two. The bandits will approximately deal 11 damage to the party (assuming every bandits could Attack each round).
The gnoll warrior will be killed in round 2. The gnoll warrior will deal approximately 7,5 damage to the party (assuming the Gnoll Warrior could attack each round and use his special ability in round 2).
The bandits have a chance, by concentrating attacks, to down on PC. The Gnoll only if he attacks a wizard.
Okay, let's take another 100xp creature - a scout.
AC 13 - Hitchance 65%
HP 16 --> adjusted 25
Attack +4, 6 damage, twice. Hitchance against AC 15 = 50%. So 6 damage per round.
Hrm. That ends worse than the Gnoll Warrior. Dies in round 1, making 6 damage.
Okay, so a 100xp encounter is no good, anyway.
Let's make it interesting.
Deadly/High encounter. 400xp budget.
Single creature ... needs to be 450xp.
Awakened tree
Ac 13 --> 65% hitchance.
HP: 59 --> adjusted 90HP.
Attack: +6 for 14 damage. - adjusted 7,7 damage.
It will take our party of 4 approximately 3 rounds to take that tree down while at the same time 1,5 party members could go down. If we account for a part member to go down in round 2, there is still a 40% Chance that the tree will go down in round 3, if not than in round 4 (not accounting for 4 damage).
Goblin rading party:
Golbin Boss + 5 Goblin Warriors = 450xp.
AC 17 (45%) & 15 (55%)
HP 21 & 10 (×5) = 47 HP & 18HP (x5) = 137 HP total.
Attacks: Boss = +4 5 damage x2, Warriors: +4 5 damage x5.
Not accounting here for advantage or.nimble escape ...
First round the Goblin raiding party makes 17,5 damage, downing at least one PC, the PCs take out 2 Goblin warriors.
Round 2: Goblins take out another 2 PC, PCs take out another Goblin Warrior ... and TPK.
Damn. Without advantage and nimble escape.
But to be fair ...
A Green Dragon Wyrmling (450xp) would also TPK our first levels party in round 1 with its poison breath.
But the problem could be Level 1, which is quite deadly.
Let's do level 4.
Level4 party
Hiigh/Deadly XP budget: 2000xp.
Single Creature: Earth Elemental (CR5, 1800xp)
AC 17 (Hitchance 50%)
HP 147, adjusted 294
Attack +8, 14 damage x2 (70 Hitchance) --> adjusted damage 9,8x2 = 20.
Puh, a slog fest. If no PC goes down, it will take 5 rounds to take out the earth elemental.
The Elemental will take out a PC every second round. Off, at least 2, even 3 PCs could go down if we assume average rolls on everything.
Now ... 4 Gricks (450xp x 4 = 1800xp)
AC 14 (65% Hitchance)
HP 54 (×4) - adjusted 83 x4 = 332 HP.
Attack: +4 for 9 and for 7 damage.16 damage, adjusted to 8 - 4 times.
Hrm equally hard. So, the PCs will struggle to take kn Grick down jn round 1, whe the gricks will take one PC down in round 1. It again would take 5 rounds to take them down if no PC goes down.
So Round 1: one PC down.
Round 2: One Grick down one PC down
Round 3: 1 Grick down.
Round 4: 1 PC down ...
Okay, that could easily be a TPK.
So at first, I would start with moderate encounters in 5.5e, then go from there and see how well the party performs. But damn, high encounters seem really to hit harder, even for rested parties.