I have a project for PC and i want to publish it on Steam.How I'm doing it solo, I guess that the best option is to download Free Art(I'm programmer).Can I have any problem caused by downloading the models from OpenGameArt, Unity Asset Store or openclipart.org ?Do you recommend it?

Regarding the business side, it's perfectly fine to use off-the-shelf assets.

But:

-Your assets should all be artistically coherent and they should be placed in a way that makes sense.

e.g. don't put cartoony zombies together with realistic ones and don't put British telephone boxes together with US post boxes.

-Don't use the demo map as a level in your game.

-The Unity Asset Store has assets which can be dragged and dropped right out of the box.

While free art isn't always game ready and you will likely spend more time trying to fix UV maps than developing the game. I had to scrap some good looking free houses because they didn't look good under lighting and had weird shadows.

Isn't like unprofessional writing a lot of credits on the credits of the game?And what if the authors change the license and want money from my game?

All the assets on these sights have their particular licence clearly stated and usually bundled in the zip with the assets. Provided you stick to these licence rules then you won't land in any legal hot water.

However the problems a lot of games have with using third party assets are:

Mixing and matching of assets. So for example pixel art from two different places that use a different color palette or even mixing vector art with pixel art.

Assets being the wrong scale so get resized by the programmer who has no idea about scaling pixel art.

Really good third party assets that become popular within the indie scene so that you have multiple games all using the same art.