I'm just coming back after about a 8 month break from SINs. I remember leaving due to the poor state of the multiplayer game where literally everyone and their grandma spammed nothing but carriers and flaks and stationary defenses were completely worthless. I heard things have changed so I decided to give it a shot.
The carrier spam has pretty much been killed off though you might see carrier capital ship spam. (They start with 3 strikecraft at Level 1 and can get up to 8 at Level 10, 11 for Advent since extra squadrons are one of the passive abilities.) Of course some people are going to spam them out but it isn't hard to counter.
Most people are playing Entrenchment (including most of the regular players), in which case starbases make very worthwhile stationary defenses. People will also place turrets and repair bays at strategic locations.
What are the power builds now? What's changed from the days of LRM spam and carrier SC spam?
You'll probably definitely want to have a carrier capital ship. One common tactic is a double Sova or double Halcyon rush, which brings you 6-10 enemy strikecraft. Halcyon brings anti-strikecraft push and Sova brings the steal income ability and the missile platforms. Also, Light Frigates have been buffed a little bit which makes it much harder to LRM spam early in the game because the light frigates are better able to fight them (still countered by them) but do a good job of killing scout counters such as flaks and a mass of cheap scouts can kill LRM spam. So, it's possible to see lots of scout and light frigate spam (no research required) though lately I have seen mostly the double Sova/Halcyon spam.
Are capital ships still crap and underpowered? Is making several of them actually a decent idea now? (4+)
Some capital ships were buffed. I already described what happened with carrier caps and it seems like most people build a carrier cap early on now, either as their first cap or their second. For Vasari, the Kortul was buffed and is now a formidable fighting cap. For TEC the Akkan received a huge eco-buff and is probably what you should start with. It gives free extractors (1-2-3 as colonize is upgraded) but most importantly it gives a mineral extraction rate buff of 33, 66, 100% as the colonize ability levels up which is huge and it lasts for a while at a colonized planet. As a result of the Akkan and Sova buff you'll see fewer Marzas and fewer Level 6 Marzas. The only notable Advent capital ship buff was the Halcyon, but since that was such a great cap before everyone builds one now (at least as a 2nd cap). The Progenitor colonize bonus was nerfed from 20/40/60 to 15/30/45.
Is Advent still ridiculously overpowered? TEC still underpowered?
Advent still has a slight edge but TEC is certainly viable; the edge isn't that great. Vasari gets screwed early on because it's scouts and light frigates suck compared to Advent (best for that) and TEC. However, Vasari has the most useful starbase and lots of people still play Vasari.
Are non-Vasari Starbases worth getting now?
Advent starbase? Not so much. TEC starbases are scary because of the possibility of the big red button that will wipe out your entire fleet. I think they are definitely worth getting.
Please feel free to add anything else that was an important change since the beginning days of entrenchment.
The game has noticeably somewhat higher online player counts now and some new players are around. Most of the pros are an Entrenchment where you'll find the 4v4 and 5v5 games. I'm personally trying to get people to obtain and play custom maps which is much easier now because I've learned how to put the map files into an auto-installer files, allowing for 45 second installs where no one needs to find their hidden Application Data folder or move files around or even close Sins. See https://forums.sinsofasolarempire.com/372237
