The Credit Karma (VantageScore) and MyFico simulators do use the profile data they have on record as a baseline for simulations. In the case of CK, it uses your Transunion data. The only time I had access to the MyFico simulator was when I purchase a report and access was only for 30 days as I recall.
A primary drawback of simulators is the limited number of factors they can look at in a "what if" scenario. For example, if you have $20k in credit card debt and pay off $15k, the simulator does not ask for how much pay down on what card(s). Therefore the simulator cannot properly account for # of cards with balances or individual card utilizations - only aggregate utilization.
For profiles with multiple derogatories such as lates and collections, the simulators lack the ability to analyze impact of changes in specific subsets of the delinquencies. Thus, their accuracy becomes even more unreliable in these cases. Here is what I see adding lates on CK: Whats up with the VS3 score boost at 90 days?
| # accts late | no lates | 1 month | 2 months | 3 months |
| 1 | 826 | 732 | 713 | 741 |
| all | 826 | 674 | 627 | 693 |
The aggregate credit card balance (utilization) simulation is more insightful and somewhat accurate based on comparing to my actual VS3 results. I can't recall if MyFico has a total CC balance simulator.
I still see that same score spike but, this time peak score is around 5.5% vs 4.7% for a simulation last year. Unfortunately, my high score potential has also dropped 4 points (due to fewer total accts?) 
| Ag Utilization | VS 3 Score | Ag Utilization | VS 3 Score |
| 0.00% | 829 | 10.55% | 824 |
| 0.03% | 829 | 10.92% | 823 |
| 0.40% | 829 | 11.65% | 822 |
| 0.77% | 829 | 12.21% | 821 |
| 0.96% | 827 | 13.13% | 820 |
| 2.07% | 826 | 13.50% | 819 |
| 2.63% | 825 | 14.62% | 817 |
| 3.70% | 824 | 15.54% | 816 |
| 4.44% | 823 | 16.28% | 814 |
| 5.04% | 822 | 17.39% | 812 |
| 5.74% | 831 | 18.32% | 810 |
| 6.11% | 830 | 19.43% | 808 |
| 6.66% | 830 | 20.37% | 806 |
| 7.26% | 829 | 23.31% | 800 |
| 8.33% | 827 | 24.98% | 796 |
| 9.25% | 826 | 27.76% | 789 |
| 9.62% | 825 | 30.91% | 783 |