She doesn't seem all that good. What am I missing? My wife usually plays two Rebels, but I don't think Saska would be all that good.

She's strategy-maximizing. You don't really "play" her any more than you play Gideon. She makes everyone better.

• Battle Technician doesn't require an action. It essentially lets Saska bank Strain to be used later.
• Unstable Device: Crazy useful. How many times have you seen a ST group early on each with 1 health? Yellow dice have a 2/3 chance of at least 1 damage.
• Tool Kit: Skill checks are annoying. Adding 1 surge to any skill check is amazing. Side note: She might be the only character that gives the Rebels a chance at winning "A New Threat".
• Structural Weakness: Not bad, there is attacking a lot of Doors and Objects in this game, and they tend to have a lot of defense. Defense is your tax for attacking, you want to pile on as much damage per attack as possible.
• Energy Shield: Makes herself a tank, could help win missions.
• Gadgeteer: Increases her usefulness a ton. Also allows her to attach mods to weapons that weren't meant to have mods attached to them (insanely useful). Give her the DXR-6 with Tactical Display and let her shred everything.
• Power Converter: Pretty useful, consider it doesn't have Exhaust. A lot of weapons are non-optimal for every situation. But utility probably isn't high for 3XP.
• Remote Distribution: Expensive, but increases the usefulness of tokens.
• Adrenaline Injector: Pretty darn good. You essentially supercharge your entire team.

I wouldn't use her in a 2-Hero mission because you're giving up attacking power, but in a 4-Hero she'd be incredibly useful.

Her best abilities:

Tool Kit

Unstable Device

Gadgeteer

Adrenaline Injector

Remote Distribution / Power Converter

What about a 3 hero mission with Biv and Fenn?

I think the big problem is FFG touted her as a support character. And while she is, she is more of a support/dps hybrid than full support like Gideon.

The extra mod from Gadgeteer combined with the surges from device tokens can propel her dps be on par with Gunslinger Jyn. Meanwhile, giving out surges early campain, and extra movement + focuses make her a great team player. She is extremely good if you can get Tech Goggles from her personal quest.

I was unsure about her at first, once I played her and realized she's a hybrid, then she really grew on me.

Biv / Fenn / Saska wouldn't be a bad group. Ideally you'd go with 4 people since 3 has some disadvantages (less starting Strain, Heroic only grants 9 health, and if the IP gets Weakness Revealed he can assign it to the player with 2 activation tokens).

Fenn's tactical movement would help keep Saska close.

Biv for all intents and purposes is a melee character so he won't be near Saska most of the time. Saska is really best with a long-range weapon, since she'll be resting a lot.

Saska will be resting every other or every third turn, so most turns she'll only get one attack. Get her the E-11/DL-44/E-3 Carbine and load it up with mods with Gadgeteer. Get her something with some range so she won't have to move in close to take a shot (Marksman Barrel).

The problem with the 2 mod weapons with Saska is that you will have troubles getting value out of the extra mod slot from Gadgeteer. Also, because of device tokens, she likes weapons with strong surge abilities (like DL-44), red dice (like deathhammer - which have low surge chance), or both (like the DXR-6). Marksman barrel is a pretty good use of the extra slot as she doesn't want to be up front due to being pretty squishy. And while, humorous thematically, Plasma Cannon w/ 2 mods hits extremely hard.