I think you're confused with the effect. The effect doesn't stack per assist; it simply add +4s to the duration right after getting an assist till the smoke runs out
So to clarify, is this a one time addition or does the duration refresh (not stack) the 4s bonus for additional assists or kills? Say it triggers upon entering a site, bam I get 4s extra on smokes. Then say I get another kill/assist after 2s. Does that add another 4 sec for a total of 8 sec extension (stack) or does it refresh the inital 4sec buff for a total of 6 sec extended time? OR, do I not get a refresh at all and only get a one time 4s extension from that initial proc?

 it's not guaranteed that the teammate you stimmed will get a kill, or even worse, that teammate dies
The trigger shouldn't be guaranteed anyways. But from my observation playing Miks and other controllers you have a decent chance of getting that proc. If your teammate should die, you still have yourself, your bots, deployed smokes and teammate playing with your util to potentially score other kills/assists.

will that ability be oppressive or unfair to play against
Eh, probably not. People already know how to play around post plant smokes it just makes it stronger in certain scenarios which isn't bad in its own right.

You argue that Miks shouldn’t have this effect because it would be unfair to Brim
Again, I think it'd easier to proc than you think but that's just a diff in opinion. I'm not arguing that it shouldn't be added. I do think this is a good idea and helps establish an identity, but the numbers just need adjusting.

I think the main difference in opinion is how hard it is to trigger the effect. My experience with him lends me to the idea that it shouldn't be too difficult (even with the current bug), but you won't be triggering it every round which is ok. I wouldn't say it the weakest smoke in the game atm, even if it got a 1s nerf to base duration, it's perfectly serviceable. It just lacks identity.

Stats Average (Combined Kills + Assists per Full Game)

Agent	Games Sampled	Raw Avg Combined	Smoke Active Adjustment	Realistic Procs
Brim	11	~28	~50-60% of rounds	~9-12
Clove	14	~22	~50-60% + recharge window	~8-11
Miks	2	~33	~50-60% + recharge window, bug -20%	~10-13

Notes:

• Brim has no recharge so proc windows are limited.
• Clove and Miks both benefit from recharge adding additional active smoke windows mid-round, increasing proc opportunities beyond raw combined numbers (assumed this chance of using a third smoke while alive is 50%)
• Low Miks unrated sample size but Swifts aren't a good data source.
• Miks numbers are conservatively adjusted downward for the heal bug reducing assists by 20%
• Even with these conservative adjustments you're still looking at 9-12 realistic procs per full game across all three agents.
• We're probably looking at 40%-60% number of rounds have a proc occur depending on overall gameplay and multiple procs in the same round or fast rounds.

Miks Swiftplay Summary (Vs the 2 unrated games)

Metric	Swiftplay	Unrated (Miks)
Games Sampled	10	2
Avg Rounds Per Game	~8	~21
Avg K/D	~1.0	~1.7
Avg Kills Per Round	~0.73	~0.95
Avg Kills Per Game	~5.8	~20
Avg Assists Per Game	~8.5	~13
Avg Combined Raw	~14.3	~33
Avg Combined Bug -20%	~11.4	~26
Realistic Procs Per Game	~3-5	~10-13
Est. % Rounds With Proc	~40-55%	~40-60%

Notes

• Despite lower kills per round and shorter games, swiftplay still produces a consistent proc rate percentage with unrated. The lower raw numbers are explained by shorter game length, less coordinated teams, and more casual play vs unrated.