Free Hashing Power, what is the catch?

As most people into cryptocurrency, I do not have the high amount of capital to invest.
After setting myself with a budget around 500 USD, I will have to do my due diligence where and what to invest in

The amount of reviews, user experience related to withdrawal are two factors deciding when to invest in my case. In my current state, I have invested into HashFlare and BitConnect.
My balance is accumulating by using a compound strategy and reinvest from my balance.

The actual result will have to come but with these programs you will have to consider time before hitting the ROI forecast (approx 2-3 Months)

Free Hashing Power

There are many articles around scams/ponzi schemes and sites offering "Free Hashing power" seems to be in this bucket.
It does not necessary have to be a scam as they use it to attract more people to their websites
They have to differentiate on price as the giants of cloudmining attract most of the investors

So whats the catch?

I have signed up for two sites that are offering free hashing power:

• Coinmix
• Sea Mining

After monitoring them for two days, also here my balance has been accumulating. Not as fast as with HashFlare as there I use my balance to buy more hash power. As with most sites, you can only peek in after sign up and these are my observations and considerations:

Coinmix provides a real-time overview with multiple coins to invest. It has an easy interface where you can turn on the dials by allocating power to the currency. With the free hashing power, it will take a while before you can withdraw.
Per currency there is a minimum amount before you can withdraw

Sea-Mining offers a clean interface and dashboard with an hourly profit update.
The catch is that you can only start withdrawing when you have 0.003 BTC and you have purchased the minimum of 100 GH/S. As you receive a low amount of free hashing power, it will take a long time before you will be able to reach it

Conclusion

Offering free hashing power has been a creative way for startups/companies to attract new users to their site
They are actually updating your balance slowly although withdrawal can only be done when hitting a minimum threshold.

They are obviously hoping you like the community website and are willing to reinvest your profit or deposit
Before investing, please keep in mind it could take months before you withdraw unless you are willing to invest coins in to these companies offering free hashing power