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So mu understanding is that you should take mercs with Cygnar because mercs just have units that outshine Cygnars guys . So...

Which units/solos/jacks in Cygnar should not be bought because a better Merc unit/solo/jack exists? and what is the Merc unit/solo/jack to get instead?

Generally you won't find mercs that do the exact same things better, more that the specialties and benefits of merc units are easier to use than some cygnar units.

Some of the most popular mercenary units are character units such as: Boomhowler & company (even though they are filthy trolls), Cylena Ryfael and the Nyss hunters, Alexia Ciannor and the risen, or Lady Aiyana & Master Holt.
A couple of other noncharacter merc units that see a lot of play are horgenhold forgeguard and press gangers.

Some of the Cygnar units that struggle to see play are trencher infantry, standard stormguard (rather than silverline, which see limited play), and both of the trencher light artillery units.
Precursor knights, silverline stormguard, trencher commandos, and long gunners have niche builds but probably shouldn't be a new player's first units.

Many of the most popular mercenaries are solos, which take up fewer points for more utility. The list of good merc solos is very long.

It really depends on the caster. Some caster's abilities rely heavily on in-faction models. Some do not. An excellent example is Stryker3's feat. It only effects in faction models.

The other thing to remember is a lot of the popular merc units are also character units. Which means in a tourney setting you can't take them in all the lists.

I also don't think you can say, replace this faction unit with this merc unit for the most part. You need to decide what you need a unit for or what function they are going to provide and then choose the best one.

And most of the Merc solos are for support. So again, you need to look at your list and see what you need to support it. For example a shooting caster can benefit greatly from Reinholdt. But other casters get less benefit from him. A lot of units can benefit from Rhupert, but sometimes Jr is a better way to go.