RH, for what it's worth I'll share experience with my Poseidon, I have about 500 miles on it. Not sure which one you are looking at, I have the X Ambition.
1.The claimed weight of 25lbs (W/O Pedals) was pretty accurate.
2. The frame welds look fine, they are not perfect, but no issues.
3. The bike was very easy to put together, although I have been building bikes since 1974, so if you have NO experience with this you may want some help.
4. I did have to adjust the brakes, not a big deal 3mm Allen wrench.
5. I also had to adjust the derailleur, again not a big deal...
6. Stock tires are 40mm Kenda, I swapped out for 43mm Panaracer GravelKing SK's (much lighter and I wanted the added width for bikepacking).
7. Stock cassette is steel, I switched out to alloy (8 oz lighter).
8. Seat is personal preference, I sold stock seat and put on a Brooks C17.
9. I got the bike when it was $800, I have $1,100 into it.
10. They do offer a Thru-Axle upgrade for $300, which includes a full tune of the bike and tubeless set-up, this would be worth it.
11. With Shimano SPD Pedal E-PDM324 and (2) bottle cages, the bike weighs 25.05Lbs; I'm happy.
12. No issues with anything else, if there is a much better bike out there for $1,200, I'm not aware of it; especially with Aluminum frame & Carbon fork.
13. Looking at reviews, I think the Advent X group is as good or better than other groups in the same price range.
14. All that being said, going to your LBS will get you full service... I can see the advantages of that.
15. The brakes are spongy, I will upgrade to JagWire housings, <$35.