I run
The 1650 Psalter website and co-led the Scottish Metrical Psalm Sing at the RP International Conference last year. It should be obvious where my preferences are.
That having been said, I've noticed over the years a few places where the Scottish Psalter (like the Authorized Version) could be better.
You've already mentioned the second version of Psalm 136. I've also noticed Psalm 103:17:
But unto them that do him fear
God's mercy never ends;
And to their children's children still
his righteousness extends.
The AV has, "But the mercy of the LORD is from everlasting to everlasting upon them that fear him, and his righteousness unto children's children." I've noted for years that His mercy being "from everlasting" indicates divine election; and His mercy being "to everlasting" indicates the perseverance and preservation of the saints. The Scottish Psalter unfortunately "smooshes" them together here (or simply elides the former element). Both of the newer RPCNA Psalter (1973 and 2009) do a better job here. Unfortunately, they introduce a problematic rendering of verse 13 of this Psalm, omitting the fear of the LORD that is being repeated from verse 11. Oddly enough, the 1912 Psalter does a better job at both of these places (verses 13 and 17), but is generally horrible throughout. In other words, you won't find a perfect Psalter. I would say that, generally speaking, the Scottish Psalter is superior to any other in the English language on the score of accuracy. Any others that I have looked at usually introduce other problems when they try to "fix" the 1650.
As far as some of the points of criticism that you've already mentioned:
1. Less understandable --- I totally understand.

I've found this to be less of a problem since I already use the AV. If you have TBS Bibles with the Psalter in the back, they probably already come with a word list that helps to identify less common words; I've found that most of such words in the 1650 also occur in the AV. But you already mentioned that you use the Comprehensive Psalter, which also defines unfamiliar words. As far as the rearranging of words, I would recommend either reading the passage through before singing it, or reading it in the AV, or both. Usually the meaning comes out when you slow down to think about it.
Personally, I find it less easy to follow the meaning of a Psalm when it's meshed together with a tune. I precent for
The Book of Psalms for Singing in our congregation, and I've found it easier to get at the meaning if it's a longer Psalm with some concluding stanzas that are below the tune (for example, the four concluding stanzas of Psalm 32A). You can see the whole stanza at one glance, and therefore have a better grasp of the overall meaning, rather than trying to take every word as it comes, while trying to follow the tune with your eyes at the same time.
2. The meter is off --- Admittedly, that can take some getting used to. It starts with the third verse of the first Psalm: "river" and "never." When I first started using the Scottish Psalter, I had never heard anyone else use it, and I thought the lines with a -tion word that were obviously short a syllable required that I pronounce it, "Sal-va-ti-on doth appertain" (3:8); whereas most that I know who have sung it all their lives would sing it, "Sal-va-a-tion doth appertain" (simply lengthening the syllable preceding it to cover two beats). And what is worse, it's not indicated in the text at all. My preference would be to have a circumflex accent mark over all such vowels that ought to be lengthened; perhaps if I ever get around to a critical edition of the Scottish Psalter (yes, there are differences that have crept into different editions!), that could get added.
In the mean time, I'll simply note that my wife has only been singing from the Scottish Psalter since we got married less than five years ago, and she is quite used to it --- only once in a great while will either of us be thrown off.
3. Difficulty in memorizing --- I understand the desire to have a distinct tune for each selection. Right now, we have a memory Psalm (selection) that we sing in family worship (we sing previous memory Psalms as well, in order to keep them familiar), with a different tune assigned to each. Historically, though, Presbyterians would commit large portions of the Psalter, or even the entire Psalter, to memory --- even when they would know perhaps only six or fewer tunes. My guess is that they learned them as one would learn poetry, with the ability then to "plug in" the words of a Psalm to any tune. Sometimes, I wish I knew the Psalms in this way; I often find it difficult to remember the words of a Psalm without remembering the tune, or to sing a Psalm to any tune other than the tune I've matched to it.
As far as the history is concerned, Scotland had one authorized Psalter before this, used from 1564 to 1650. If you know Tim Duguid, you could ask him about it.

The Scottish Psalter of 1650 was not used as universally as the AV Bible; the Scottish Psalter was used by all English-speaking Presbyterians until the mid to late 1700s, while the AV Bible was used by all English-speaking Protestants until the late 1800s. In America, all Presbyterians used it exclusively until the introduction of Watts into congregations; originally, Watts' influence was confined to New Side / New Light Presbyterians that were influenced by the First Great Awakening. American Congregationalists used the Bay Psalm Book (which, shortly after its first publication in 1640, had several "Scripture songs" appended to it); Anglicans and Baptists tended to use either Sternhold and Hopkins (1562), or Tate and Brady (1696). Even after the introduction of paraphrases and hymns, all of the Presbyterians in Scotland, Ireland, and Canada continued to use the Scottish Psalter of 1650, until the late 1800s. As far as a "universal Psalter," it's the closest thing that English-speaking Presbyterians have. The fact that it's
still used on virtually every continent, and
still bears the authorization of every church that descends from the Church of Scotland (i.e. every English-speaking Presbyterian church), tends to buttress that claim.
Our Synod adopted a new Psalter version in 2009. I don't know when the next one will be; I can almost guarantee that it will be in the next twenty years or so. I want to use a Psalter (and a Bible!) that is not going to change with every generation. The ironic part of it is that, one day,
The Book of Psalms for Singing (1973) and
The Book of Psalms for Worship (2009) will become outdated, and their publication will be discontinued. But the Scottish Psalter will continue to be published, and continue to be used in congregational and family worship --- in a hundred years from now, it will, in its own way, be modern and relevant; while they will be literary and musical curiosities to seminary students writing papers about the history of Psalter versions in the RPCNA. In a sense, I'm just planning for the future.