Saber Alter is actually stronger than Saber Vanilla when they are both at level 100, with Saber Vanilla's only saving grace being a superior Charisma rank.
That said, she's not shitty. If nothing else her skill kit allows her to hit hard, just not in an especially interesting way or as hard as later servants can with superior buffs and better team synergy. Arturia will fit in just about any team, but she won't be the best option. #Instinct/Charisma Buff When
That said, in Sunny Order, Arturia is actually one of the best Servants you can get, because Instinct does more than spit out crit stars every five turns, Mana Burst is a general purpose beatstick beamcannon instead of a once-every-five-turn NP steroid, and Excalibur is one of the biggest guns in existence, being the Last Phantasm. Coupled with her preexisting familiarity with both Ritsuka and Kana, not being gimped by a poor mana supply, and no hangups about being caught in a death match while being teamed up with a manchild psychopath who uses guns and explosives and refuses to communicate with you... Yeah she's pretty hefty.
Unfortunately she doesn't have Avalon. I imagine Shirou returned it to her at some point, but then she returned it back to him because he's squishier than her and gets into trouble just as often so having high speed regeneration is always good, and... she didn't get it back before the incineration of the human order.
Someone's going to be salty about that, I'm sure. 50/50 on whether its Ritsuka or Saber.