In most countries consumer arbitration is either heavily regulated, or non binding. In EU pre-dispute binding consumer arbitration clauses are considered unfair terms.
American Exceptionalism in Consumer Arbitration https://papers.ssrn.com/sol3/papers.cfm?abstract_id=2265556
Arbitration can't completely remove access to the courts -- it can (and does) make it harder to get there. Case law for this sort of thing is always evolving, and so is statutory law. It's entirely possible for the courts to modify arbitration sufficiently to make the playing field more even.
As an example there are clauses written into ToS which don't allow refunds on digital purchases. These work fine in the US but are illegal in countries with stronger consumer protections like Australia. US companies do still have to obey local laws - at least when it comes to the sale of faulty goods.
I would suspect (not a lawyer) that the main issue will be jurisdiction.
As a consumer in the EU, jurisdiction in matters with commercial entities is the consumer’s place of residence. But HN/YC isn’t in the EU, and I don’t think there are any legal agreements b/w the EU and USA that are relevant here. Free trade agreements tend to include such issues, but TTIP’s well-known fate means this is left to the parties to specify in any contract.
Then, there’s a difference b/w HN and YC. HN’s relationship to users is that of a commercial entity to consumers. But YC investing in your startup isn’t. Consumer protections would not apply even if YC were in the EU.
To summarize: yes, the arbitration clause, at least of YC, would apply to the same extend as if you were a US citizen.
If you could pull that part off, I think you could take YC to court in Germany, and even achieve a favorable judgement. However, I don't see any way how you could enforce the outcome, except if YC has assets in Germany.