Realistically very few. The plaintiff carries the burden of proving the claim, but they are afforded the power of the law to do so via discovery. The defendant then has to spend a large number of legal resources refusing the claims and also providing the evidence required.

If the plaintiff loses the lawsuit a countersuit is pretty unlikely to succeed unless the lawyer participates in gross misconduct. Generally, countersuits are filed more to put the original plaintiff on the defense and don't result in a large judgement.

If you are operating in good faith, then you are pretty insular from kickback as a plaintiff

>what are the repercussions for a frivolous lawsuit?

None, hence the high price of liability baked into basically everything in America. And not just in nominal prices, but in terms of things like restricting access to spaces, restricting access to information, etc.