I don’t think telling the author what you enjoyed about their story is insincere.
If you have to lie about enjoying the game, then that’s a whole other can of worms, and at that point I would suggest asking your friends to look over your critique so that hopefully they can help you find a way to break the news but gently, and maybe wait a couple of hours before you post so that you can better piece together your thoughts.
And yes, you have to tell them you don’t like it in a gentle way. Being told everything you’ve done is terrible is an awful feeling, and if not done properly can completely destroy any motivation the author has to continuing, or perhaps ever writing again.
Does an author have to grow thick skin? Yes, but that requires papercuts over weeks and months and years before an author can handle stronger criticism, if you cut them with a sword straight away, all they’ll do is bleed out.
Is the marketplace an even worse place for reviews than the forum? Absolutely! People give one stars just because they don’t want to pay three dollars at launch! But that doesn’t mean that giving the author brutal honesty will actually help them prepare for the bad reviews. If anything, it’s just proof that they can’t write and should never do it again.
If you word your critique in such a way that the author doesn’t even want to read your critique, then you have failed your job at giving feedback by alienating the author against you.
You have to be able to show that you understand the good as well as the bad, that you can identify to the author what works and what doesn’t. It can’t be 100% negative unless the author asks for it. If you’ve made yourself and your insight valuable to the author, then you have done your job, and they might ask for that brutal honesty from the get go from now on, but to start off with it I think will cause more harm than good.
So yes, you should tell an author the good and the bad. It is helpful to also mention something else nice at the end because it really does help.
No, doing that does not make you insincere, it means you know how to make your critique come across effectively.
Yes, helping an author reach the marketplace is everyone’s goal, but that will never happen if you can’t get the author to trust your opinion, and that means getting to their level about what they can handle critically.