The less legit it is for a website to mine crypto, the easier it is for adblockers and security to block all such scripts without inconveniencing the user (less tradeoff of usability for security).

But when some sites use it and others doesn't, it makes security complicated and hacks are able to slip through easier.

Also, the more users get used to sites mining on their browsers, the less likely they will be perturbed by and reporting CPU usage spikes (which are used to determine which sites have been hacked by cryptojackers) because they are used to seeing legitimate such spikes from sites they frequent.