The semantics of audio production have gotten so much more confusing over the years, which can cause issues.
For example, when you search for "mastering online" on Google, what are you actually looking for? Automated instant mastering? Or a mastering studio that accepts projects transferred online?
And not every mastering company is making it clear which they offer these days. LANDR, and many "online mastering" sites these days are 100% automated. There may have been engineers involved in creating it at one point, but when you have your material mastered by these sites it is completely out of the hands of any person. The easiest way to know is if they say they get your mastered track back to you "instantly" or "within a few minutes" or any other such turnaround times, you know there is no human involved. Even a human being offering "mastering" that is simply running your song through a mastering plugin preset on their bedroom computer setup still requires more time than this.
Automated online mastering has certainly improved over the years, and many are starting to offer you options to indicate specific sound flavors that you prefer for your own music - which is a big plus since it adds an artistic element back in.
What they do NOT offer is any kind of experienced professional human input. If you have a special situation, or need particular things addressed, or perhaps you're unaware of something that could have been improved in the mix or a technical glitch in the sound, none of these will be handled by any of the automated online mastering options.
Automated online mastering is like ordering your food from a vending machine. Sometimes it may be really all you need - why pay for more than you need, right? But, if you want to ask questions, or get input, or have a pro engineer on your production "team", they will not provide that for you.
I recommend using automated mastering if you're trying to get something done quickly or if you're situation makes it okay for it to just turn out "good enough". You can always send the same tracks to a real live pro human mastering engineer later when it matters more.
Keep in mind this - if your music was up for a Grammy award and they ask you to submit the name of the professional mastering engineer on it as part of your required complete list of credits, who will you list as the "professional engineer"?
For your demo album, song for grandma's 90th birthday, young artists just getting started, etc., automated online mastering is a great option.
My studio offers both (just for example): https://www.crazymastering.com/mastering.html
, in recognition of the fact that people are looking for different levels of mastering help.
I wish others were more up front about what level of service they're really offering, is all I'm saying. It's not always clear.