arsonists are easily the worst. useless in melee and get kited by anything with good range. they are only good vs short ranged low hp units like strelets or unmicroed muskets that don’t go in melee/kite their animation.
corsairs lack in stats, they get sniped really easy and even when they do get in melee vs cav they aren’t amazing, just decent. being cheap and having numbers doesn’t mean much when ranged units pick you off and the payout isn’t great. their speed isn’t enough to save them. use cheaper swiss with their larger anti-cav bonus if you can or better yet, fusiliers/highlanders/black riders. they cost the same as highlanders and are too expensive for what they give.
Landsknecht are in the same boat, only they are actually pretty good once they get in melee but are too slow to actually catch cavalry and avoid being sniped by enemy ranged units. they also don’t come in large numbers like swiss or corsairs, meaning you need to put effort into slowly moving them into position where cav has to come to you without ranged support. just inferior to the cheaper highlanders/fusiliers who are more reliable, need less micro to be effective and are better multipurpose.
Stradiots, who are roughly 10% weaker then an imperial hussar without cards are also subpar. a simple stat boost to say 700hp/70 attack would still have them lower then a carded up hussar, but will help make them a competitive fortress/early industrial pick. essentially they are just too weak for what they ask. Jat lancers are on the same boat, and could use either a stat boost or a cost/pop reduction. compare to black riders/highlanders/swiss/ronin/jeager/elmiti vs their base counterparts and the difference in their stat profiles can be stark.
all the other mercenaries are at least very good in several situations and are a great addition to your army.