Gae Bolg definitely isn't a stealthy move. When he was about to use it the first time, even SHIROU knew that Archer was about to die.
I'd also like to point out that Archer would not have been able to block a second Gae Bolg, Rioh Ahries or w/e it's spelled didn't even defend the entire attack.
And if Lancer threw Gae Bolg at Berserker, he'd take out one life, and Berserker would keep charging at him and cleave him in two. I don't see spamming thrown Gae Bolgs as a viable tactic. Regardless, Lancer is my favorite servant anyway.
Lancer vs full powered Saber would probably end poorly for both sides. If Saber used Avalon to counter, the lance would still pierce her heart. Actually that'd probably stop her from countering with an Excalibur to the face. I don't know, this is a fun discussion topic. Let's keep speculating!