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
• disproportionate/uneven 

e.g. the uneven impact of the debt crisis on developing countries 

2. TYPE

• combined, cumulative

e.g. considering the cumulative impact of a series of damaging events

• decisive | direct 

e.g. The railways had a direct physical impact on the landscape. 

3. TIME

• immediate, instant | initial, short-term
• lasting, long-term | far-reaching, wider 

e.g. 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 

e.g. 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

TOP TIPS

1. Use ‘have’ as the verb. Stay safe – there are options (see below) but they have different meanings in different contexts.

2. I recommend using singular ‘have an impact’ rather than plural ‘have impacts’. Although both are correct, the singular version is the most common and ‘acceptable’.

Other verbs which collocate with impact:

• achieve, create, exert, make Variations in the interest rate will have an impact on the whole housing market. You certainly made a big impact on Carter. 
• feel The initial impact of the reforms will be felt most keenly in primary schools. 
• analyse, assess, consider, evaluate, examine, explore, judge, measure, monitor, study It is difficult to judge the likely impact of the changes on employment patterns.
• enhance, increase, maximize, alleviate, cushion, lessen, minimize, reduce, soften We are trying to minimize the impact of price rises on our customers. 
• diminish, lessen, reduce, weaken Listening to the speech through an interpreter lessened its impact somewhat. 
• lose When peace returned, the hardline message lost much of its impact. 
• resist, withstand This section explores how mothers resist the impact of poverty on the health of their children.
• be concerned about | appreciate | highlight | address | predict | reflect Architecturally, these churches reflected the impact of the Renaissance.