The Pit has various side effects that would make it unsuitable for repeated use.
First, the powers become less effective over time. Ra's mentioned this side effect to Oliver in Season 3's The Offer:
As I told you on the mountain, boy, man can only evade death for so long. The waters' powers are losing their effect on me. My time is almost over, but my legacy won't be just ash and bone, it will be history.
Second, the Pit causes a bloodlust - particularly against the person that killed the user. If that lust can't be quenched, then it will start to consume the user. Malcolm mentions this to Thea in Season 4's Restoration
The Pit contains traces of the souls of all the men and women who have bathed in its waters. And what those waters restore, they can also take. Life for life.
Thea has been struggling all season with this side effect, to the point of near death because she refused to kill.
Third, Sara was dead so long that they had to literally journey to Hell to recover her soul. Before this was done, she was a feral shell. Even after this restoration, she experiences moments of bloodlust that she's had to be talked down from.
Finally, the Pit can't be used any more. Nyssa poisoned at the end of "Restoration" in retaliation for Malcolm Merlin's assistance in restoring Sara Lance. This was a fate that she told Laurel:
I wouldn't wish it on my most bitter enemy, let alone my beloved.
As of right now, this is the only known Lazarus Pit. Only Damien Darhk is known to have any other water from this pit. Ra's mentioned this to Oliver in Season 3's Al Sah-Him when explaining the League's feud with HIVE:
When I joined the League, I was conscripted with another man. Damien Darhk. We would become the prized agents of Ra's al Ghul. We were his horsemen. And he molded us into warriors the likes the world had never seen. We were as brothers. What happened to him? He believed himself worthy of becoming heir to the Demon, but I was the one chosen. So you had to kill him. Yes. But you see, I hesitated. And with that hesitation, he found an opportunity to escape, taking with him his loyalists and water from the Lazarus pit. And he has vexed me ever since.