Ratings: One player claims their ratings seem a bit lower but then claimed it is the only place where he got a rating reasonably near his FIDE rating.
Engine users? The site claims they believe it is important to fight all forms of cheating so the moderators and automated algorithms work to weed out bad players thus making Lichess the most secure site to play on! The claim is that they are always working on improving their cheat detection methods such as automated sandbagging detection and they are working on a “computer assistance detector.”
How successful is this? Who knows? However, if someone is caught cheating their profile will include a header saying, "This player uses chess computer assistance." Here's a funny thing: this flag will be visible in public to everybody except the cheater. Once caught, their rating is reset to 1500.
Of course, no site, no matter how vigorous their detection methods are, is going to detect cheaters one hundred percent of the time, nor will they be caught immediately. After finishing a rated game you can see a "view game statistics" link to see time statistics to help you think as to whether your opponent cheated. The idea is that engine users take about the same amount of time for every move. At least that's the theory. It seems to me that all you would have to do in order to defeat that cheat detection “method” is vary your time a lot. My observation is that people I have suspected of using engines always seem to be pretty well “booked up.” If I do a post postmortem and look over one of my games with an engine, it's amazing at how far they have book lines memorized even when I play something that turns out to be a minor sideline.
All-in-all though the site is a good one and I recommend giving it a try.