As a heavy bitcoin/crypto trader, Coinbase and Gdax are one of the worst exchanges out there. Speaking strictly from a technical point of view. Not only their Front-End interface (Gdax) suffers from lag probably due to poor engineering but they also made terrible mistakes at the beginning.

One of them being a lost deposit. The address generated by the site was wrong and thus the deposit never credited. It took a couple months for support to get on this and refund the lost amount. This happened a couple years ago. So had it happened now I'm not sure if technical support will ever get back.

I also failed to contact their technical support about other issues (can't really remember) but there was no way to reach them. They don't have proper support, they just have monkeys that tries to save face on social media. Only.

The best exchange so far from a technical perspective is Bitfinex. It is surprising because it is quite shady. But it is a rock solid exchange; and it is magnificently engineered. No wonder it is the top exchange out there. And Finex has handled the recent surge much better than Gdax or any other exchange out there.