These two synonyms take certain collocations depending on:

1. SIZE

• big, considerable, dramatic, enormous, great, high, huge, important, main, major, massive, powerful, profound, real, significant, strong, substantial, tremendous
• limited, marginal, minimal/minimum, negligible | overall | growing, increasing | added | disproportionate | uneven the uneven impact of the debt crisis on developing countries |

2. TYPE

• combined, cumulative considering the cumulative impact of a series of damaging events
• decisive | direct The railways made a direct physical impact on the landscape. 

3. TIME

• immediate, instant | initial, short-term
• lasting, long-term | far-reaching, wider It is important to appreciate the wider impact and implications of this proposal. 
• future, likely, possible, potential 

4. Positive/Negative

• beneficial, favourable, positive
• adverse, catastrophic, damaging, devastating, disastrous, heavy, negative, serious, severe

5. The ‘discussion clock/thinking tree’ types

human The severest human impact on the dolphins has been the loss of habitat. | personal The personal impact of party leaders has been very important. 

physical, visual seeking to reduce the visual impact of wind farms on the landscape | cultural, ecological, economic, emotional, environmental, financial, health, political, psychological, social The environmental impact of power generation is being assessed.

You can say ‘optimistic/pessimistic impact/effect’ but I would avoid it.

It can be used in very limited situations:

https://ludwig.guru/s/optimistic+impact (only one example) 

https://ludwig.guru/s/pessimistic+effect