I’ll give some of my own thoughts and then add to some of yours.

Principles of Communism was a fun re-read for me, and I found the text much better and more informative than I remember it being from when I first read it during my initial introduction to Marxism. I read it together with the Manifesto and it didn’t really stay in my mind as that much of an important work - it was overshadowed by the Manifesto, I guess. Now, I would definitely say it doesn’t deserve that fate and has a firm place of its own in Marxist theory.

It clearly defines certain important terms and concepts and compares them to similar ones present and even prevalent at the time of its writing, and talks about many thing which we still see today, for example, the periodic crises of capitalism and the realities of class struggle. I certainly see it being a beneficial read for workers organizing in 1847 and can see why it still remains a staple Marxist text, especially one that’s recommended to people newer to Marxism.

As for some of its faults, the big thing that jumps out to me is the talk of labor as a commodity, which is to be expected since this is a text from 1847 and Marx didn’t complete his critique of political economy by then. The commodity which the proletarian sells is later corrected to labor-power which is a change in texts from 1859 onward, and which Engels explains in his Introduction to Wage Labour and Capital quite nicely.

I found the bit about proles potentially being worse off than slaves strange.

This was a quite common assessment in those times. Whether it was correct or not isn’t that clear. Slaves, while not free and, as such, not treated as people but property, were still given food, water, and shelter by the slave owners who has some interest in maintaining them as their property. Of course they were still slaves, and were treated horribly as such, especially in the US South. On the other hand, especially the early proletariat, while recognized as more human than the slaves, was largely just treated as mere labor machines, working longer than 12 hours per day for very small wages and even being worked to death in some cases. They mostly lived in horrid, overcrowded, unsanitary conditions. If they had lost their job, they had no guarantees or social safety nets and would lose their housing and be unable to afford food. Practices of jailing and then basically enslaving through workhouses of people without jobs were quite common. The condition of the proletariat improved massively when it started to organize and gain some ground through class struggle. So, while from our modern perspective a statement like that is clearly absurd, things weren’t so clear back then. The slaves, while their position is clearly worse in general, could have had some better outcomes than the early proletarians.

The way he described the ‘civilized’ Europeans spreading technology to the ‘barbarous’ nations is definitely problematic in its framing.

This is a feature that oscillates back and fourth somewhat in Marx and Engels, although they seemed to form a good anti-colonial line by the end. They mostly - and by Engels’ own admission, perhaps erroneously - focused on the economic side of class struggle, especially in Europe. In terms of solid theory, this is a problem solved by Lenin who fully expands the theory of class struggle to encompass all aspects, and not just the binary of proletariat-bourgeoisie which is dominant in the West. The seeds of this are present already in Marx and Engels, but aren’t conclusively worked out. For a full discussion on this I would recommend checking out Losurdo’s Class Struggle, specifically chapters 4, 5, and 6 - although the entire book is an amazing read.

P.S. - The link to the Principles of Communism in your post just leads to this post and not to the text itself.