Stalk means that you generally can't fire on waywatchers with artillery until they are in range themselves. In contrast to sisters of Avelorn, waywatchers have stalk, fire on the move, 360 shooting, and loose formation (as well as slightly more range) which all add together to make it much more difficult to use the tools mentioned above to shut them down.
The wood elf archer comparison is apt, but the focus on waywatchers is misplaced. Yes, they can potentially destroy valuable units quickly, but they can only do so if they are not wrecked by magic or artillery and the rest of your army can keep infantry and cavalry/chariots/monsters off of them so that they can fire. To answer, the single player question, they are bad because they are not cost effective. But being a semi-viablr option in a couple of matchups is the sign of a bad unit not a good one. In multiplayer, sisters are usable in a couple of matchups. I really do not think they need to be any stronger unless you are also gonna increase their price a fair bit. With their Martial Prowess active they go up to 50 melee defence for crying out loud and they also have pretty darn high WS and good melee attack. I mean, they are archers but their combat stats are actually better than a lot of melee troops. They are pretty much one of the strongest units you could possibly get for 1100 gold.
#Sisters of avelorn total war plus
Of course, there's a small selection of targets which they aren't great against, stuff with copious amounts of missile, magic or fire resistance (such as Louen with his Lions Shield active, Irondrakes or Stone Trolls) but they really have more than enough killing power, plus weakness to fire and physical resistance are a lot more common than fire or magic resistance (with the exception of the entire dwarf roster, of course). Considering that they are already the second strongest archer unit in the game and can under certain circumstances also outperform the strongest one, I think they are quite alright. 2K A Total War Saga: Thrones of Britannia.844 A Total War Saga: Fall of the Samurai.