RE : for acts like snsd, is it true that only taeyeon is doing well as a soloist (singer)?

Yeah, it's weird that none of the other members managed to have successful solo careers.

Like I think Seohyun and Tiffany did pretty good. Tiffany I think had the 2nd best solo debut, but then she started releasing English songs. Maybe it's because Taeyeon is the only one who has been consistent.

I don't know enough about 2nd gen, but I know for 3rd gen, I think Mamamoo, BP, and Red Velvet have all had multiple members have decently successful to hit level solo debuts.

Mamamoo has kinda fallen off since Hwasa's last release didn't do great, and we'll have to see how the public responds to BP and Red Velvet's 2nd releases. Though, we still have to get Jisoo's solo first.

It's unfortunately true but at the same time rest of the members barely have any opportunity as singers compared to most artists.

Tiffany, Yuri, Seohyun all released mini albums not to long after Taeyeon and were touted as the next true soloists of SNSD. That was short lived as they basically stopped straight after: SM pissed Seohyun of so she left, Yuri went to acting, Tiffany left and currently more into TV and acting.

Sooyoung and Yoona were actresses from early on, never really went the solo route (Yoona had a couple original songs and the rest were covers), Sunny went for TV.

Hyoyeon is the only other member that pursued music full time but she's never really been that popular. Besides she's going on a different way to Taeyeon or any other soloist, she is a DJ who makes and performs music for club tours worldwide. Apples to oranges

So basically Taeyeon is the only member that followed through with a proper music career, the rest either stopped very early or never really tried.

isn't SNSD the only act Taeyeon is in?

aside from Tiffany and Taeyeon, the rest of them didn't really aim for a musical career

Tiffany aimed for West which was a big gamble