They are not illegal, but you also have to make sure that the one you are interested in is legit. After all, they are going to trade on your behalf and use an exchange API to access your account, so you need to assess pretty well the reputation of the trading bot, and consider just how much you are willing to let it manage (that is to say, the amount you can expose).

It would be nice if others with hands-on experience could bear their testimony here. Unfortunately, I am not in that situation, but a couple of years ago I did take some time to look into them.

Bearing no relation to it whatsoever, and without intending to vouch for it, I did find it interesting to read through a thread here on the forum at the time: [GUNBOT] Trading Bot with 136 exchanges available. Even if you are not interested in that particular software, reading through the thread and the wiki is pretty interesting, and let’s you get a view on what this sort of software can do (you’ll also need to figure out what it can’t and the associated risks).

Just don’t assume that it’s plug and play, click, and get rich. The algoritms need to be selected and parametrized according to your needs, and sudden market sways may not be played well if parameters are not thought through.