Here are my full thoughts on the subject - hopefully not too lengthy!
I'm quite surprised it's considered outdated by any means. Since starting, I've found the layout to be extremely well balanced, at least for my tastes. One of the biggest upsides, I'd say, is that my index fingers simply haven't gotten tired from typing yet - so far, the only fatigue that I've encountered was on my left ring and left middle finger. I suspect this is mostly due to the stretching of the ring finger to the 'W' position more often, which I haven't ever changed until this layout which has 'G' there instead (aside from changing it to 'y' for a short time when dabbling with creating my own layout manually).

Here's a few reasons on why I ended up choosing this one over Colemak and its variations, MTGAP, RSTHD etc.
1. I don't need the layout preinstalled on a computer, as I just switch to qwerty and would prefer to keep my speed sharp anyway.
2. I wanted the shortcut keys Z,X,C,V to remain easily accessible to the left hand, although I like to shift V down one key to the right to take advantage of the good positioning for a 'D-H mod', moving higher frequency letters to the qwerty V and M positions.
3. I may be an ergonomics enthusiast, but even I didn't want to bother re-learning comma, period, and forward slash positions. It definitely helps in terms of switching back and forth between layouts. Now that I think about it, I was really close at one point to just choosing MTGAP 2.0, as learning the new muscle memory for punctuation wouldn't have been too hard.

Okay, seems normal so far...
4. I wanted a higher frequency on the index fingers, but not in the centre columns. This is why I moved my high-frequency letters I & L to the bottom row, I find them easily accessible there (And many colemak-DH users would probably agree). As for the 'higher load' for index fingers, I know they are extremely dexterous and strong. Maybe not thumb strong, but unless you want to use the AltGr key in a wide mod or shift your hands an entire row upwards, higher thumb use isn't really plausible for standard keyboards. If you want to see something funny, I found a video where the dude is pressing the *spacebar* with his right index finger!! And his index finger somehow still wasn't getting tired from that.
Video here

5. I wanted less load on the pinkies, but not too little. There's a lot of layouts that go crazy with home row pinkies, Colemak being one of them. Unless I had an ergonomic keyboard with thumb clusters to move [ ] - = \ keys somewhere easier to reach, I wouldn't want letters like O on the far right (even then, I would be averse to it, because I would want to use the same layout on a regular keyboard as well). I've tried that for a short time and it made my right hand sore quickly in my experience, although that's not to say your hand wouldn't get used to it (as the left pinky has with 'A').
H, in my opinion, is one of the best letters for the right pinky. Higher frequency and my hand gets sore, lower frequency and it's underutilized. Additionally, it's quite volatile - start programming or remove 'the' from the equation and the frequency takes a nosedive (easily noticeable by the discrepancy between H's text frequency and dictionary frequency). This is especially notorious for anyone who wants to use the same layout for another language, many of which may as well send H to the shadow realm (see French, Italian, Spanish frequencies). Colemak DH moves the H key to the 'M' position, but I really think that doesn't do it justice - both the key position and the letter itself (especially with the natural tilt on non-matrix-like keyboards, the qwerty 'M' position may as well be the best non-home row position on the keyboard).
I was also never a fan of the 'a' position, which I only was fine putting up with back when I was using Minimak (I tried a lot of variations from Minimak-4 to my own 14-key change variation. If I wasn't interested in this stuff, I would go with a Minimak-6 key variant, but that's beside the topic).
BEAKL 15 was a contender, but I really didn't like the letter 'L' positioning to the right middle finger's bottom row and 'H' positioning to the left ring finger's top row, as well as a couple of other things. Not that the layout is poorly made, it was a necessary sacrifice to lower the centre columns usage and almost entirely remove pinky usage.
Moreover, I've used some magic to permanently change my laptop's capslock to backspace without software (via this guide if anyone is curious: https://www.youtube.com/watch?v=PlPoG7MAt_g) so I wanted something lower frequency for my left pinky to compensate.

Obviously, CarpalX isn't perfect either (according to the steveP's fork of the keyboard layout analyzer with typing test text I got from this forum - some sort of article about covid19 from a guy named John Ward - ColemakDH achieves 69.10 while QGMRJY gets 61.98, about 90% as 'efficient'). My major focus in picking this layout is to find something that is extremely comfortable to me, while typing quickly is an added benefit.
Sqarology's Evotron 23 layout posted on this forum recently was really something great, although being exclusively for the Ergodox-EZ.
QGMRJY is essentially the middle ground I chose to fit my needs and be usable with any keyboard.

I may even prefer a layout with the letter U on the right pinky and C on the left pinky to go further in that direction, but I personally haven't seen much like that. A sort of BEAKL with heavy column usage for an easy DH mod.

Thanks for the response. I do understand that CarpalX is not simply 90% as efficient as Colemak - I was trying to point that the layout is roughly worse to type on in terms of finger movement, etc. because it forces higher frequency keys together onto the index fingers, but not by too much that it's worth throwing away.

Could you point me toward the layouts you mentioned (ISRT, SIND, and Boo)? I'm not familiar with them and could not find any information about them either.