Plaintiff's lawyer here:
Arbitration clauses are...OK. Mandatory arbitration is still less than ideal, but it's workable. I'll come back to this in a mm,net.
The real kicker in mandatory arbitration is the class action waiver. While you don't need a class action waiver in an arbitration clause, 99.99999% of the time you will have both. This is a huge deal because if you can't bring a class action, you have no practical recourse to enforce your rights in the most common circumstances.
Let's say Dropbox decided to add a $1 "lol we need the cash" charge to your bill without your permission. And they did it to all of their millions of users. Dropbox makes millions with a blatantly illegal act. But only a fanatic will sue or bring an arbitration over $1. Consumer vs business arbitration filing fee for the consumer by itself is around $250. So Dropbox goes unpunished and you go uncompensated unless one of the government bodies act. If one could bring a class action, however, the millions could combine their $1 claims into something worth bringing and something that would hold Dropbox accountable. You might ultimately recover only 25 cents for your claim, but this did more than bringing your $1 claim would have done.
It flips what should be the calculus. By adding a class waiver, companies can now compare what they stand to gain from fraudulent practices to what they stand to lose on an individual basis should one of the defrauded individuals sue. It doesn't take a mathematician to know how that formula shakes out...especially with the government regulating bodies and enforcement agencies being overburdened as is.
Going back to arbitration alone...it can be fine, it can be bad. Studies show it's less friendly to plaintiffs than court, iirc. Part of the problem is if Dropbox elects to use JAMS arbitration in their contract, JAMS arbitrators are going to be incentivized to rule for Dropbox, lest Dropbox changes their TOS to require a different agency, and JAMS loses their business.
In a nutshell, arbitration clauses, when combined with class action waivers, are one of the most harmful modern developments for consumer rights. We might get some relief with the CFPB looking to ban any financial company that receives government contracts from including a mandatory arbitration provision in their consumer contracts (or something like that) but that's only one step in one industry.