Tainix is no good.

I geared her up and leveled her up and even considered 5 starring her when I asked in my clan discord channel and most of them used her as a 4*chicken. I would do the same; made the mistake of equipping her with decent gear

You could use her to

1: Use her in FW if you have no other choice but if you do; then

2: Use her to guard your vault

The only other healer from the Demon faction I own is Vilespawn. So there is some need for me to build Tainix.

It  is  possible  to  beat  faction  wars  demonspawn  without  a  healer-  because  there  is  only  five-  Fellhound,  Vilespawn,  Abyssal,  Duchess,  and  herself.  You  can  use  Lifesteal,  Lifedrinker  and  Shields  to  keep  your  team  alive.  Otherwise,  yes,  her  only  main  use  is  FW  as  a  healer  and  Decrease  Speed  champ.

Overall,  it  depends.  What  demonspawn  champions  do  you  have?

https://ayumilove.net/raid-shadow-legends-tainix-hateflower-skill-mastery-equip-guide/

Anything that is too hard to figure out with a champ to me ain't worth it.

I  would  rank  an  Average  Champion  with  score  of  3  out  of  5.

I  would  rank  Tainix  with  score  of  3.5  out  of  5.

This  is  why  players generally don't use  her  in  alot  of  area's.

Their  is  alot  of  better  options vs.  Tainix in  the  grand  scheme  of  things.

Tainix  gets  mainly  used  in  Faction  Wars.

Tainix  healing  competitions isn't has  high  as  her.

-  Abyssal  is  a  3  at  healing

-  Fellhound  is  a  3  at  healing

-  Vilespawn  is  a  2  at  healing

If  you  have  60k  HP  on  Tainix,  She  can  do  1,800  Heal  per  hit.

Keep  in  mind,  She  hits  twice  so  it  be  around  3,600  Heal.

If  you  add  Masteries,  you  could  jump  to  4k  Heal  most  likely.

Sycl  of  Drakes  does  a  10%  heal  based  on  allies  HP.

Most  generic  Nuking  hero's  will  have  HP  amount  of  30k  -  40k.

If  you  think  about  it,  Sycl  heal  will  be  around  3k  -  4k.

The  Healing Tainix  does  is  certainly on  par  with  Sycl.

Some  might  even  say  Tainix  healing  is  better  vs.  Sycl.

The  problem is  you  have  to  jump  thru  a  bunch  of  hoops.

Tainix  has  a  bunch  of  restriction which  need  to  be  met  in  order  to  heal.

Sycl  has  no  restrictions  -  She  only  needs  to  take  a  turn  which  is  easy  to  do.

Obviously,  we  can  argue about  Tainix  all  day  long.

Some  can  say  you  should  build  her.

Some  can  say  you  shouldn't build  her.

Both  sides  will  most  likely  have  Valid  Arguments.

In  the  end,  it  will  come  down  to  what  you  want  to  do.

The other Demons on my account are: Peydma (lvl 60), Excruciator (lvl 1 so far), Erinyes (2 of them, so I could still use one after I did Rhazin fusion), Marquess (lvl 40), Vilespawn (lvl 40).

None of them exept Peydma seemed good enough to me to get lvl 60. Maybe I'm going to invest in Excruicator, but I didn't find the time to do so yet. Had to level up good champs from other factions, Uugo and Gorgorab.

3 weeks away from Drexthar. 

Until you can smell Lydia don't upgrade for FW, imo. If the champ has no use elsewhere you likely have a better use for those chickens. Sometimes the best choice is not to 6star anyone on your roster. 

Also got her recently, she is still level 1 and in my vault she looked trash to me.

Based  on  what  you  said,  I  would  wait  on  Tainix.

I  wouldn't use  her  as  a  chicken.

I  would  just  save  her  in  the  vault  till  you  get  closer  to  beating  Demon  Spawn  Faction.

There are seven actually; you left out infernal baroness and achak.