Mech mods are basically as complicated as a hammer and just as subtle.
The pros are they have no electrical components to break so, as long as they are kept clean will work - and as there is no circuitry/wiring to sap or delay the power delivery they hit instantly.
The cons are - everything else...
Because there are no electronic protections a hard short can/will make the batteries vent and spray red hot toxic chemicals.
You absolutely must be aware of Ohms Law and build accordingly - building too low can/will make the batteries vent and spray white hot chemicals. Building too high will result in a feeble underpowered vape, you must be able to put ohms Law into practice and build to a suitable resistance to suit the wattage you need for the atomiser you are using
You must be aware of battery safety and exercise it rigorously. A damaged wrap causing a short or exceeding the Amp CDR rating can/will make the batteries vent and spray white hot chemicals,
In any of the above instances yoiu will probably be holding it up to your face and you will probably get a face full of the same white hot chemicals.
They aren't safe to carry in your pocket - if you accidentally pocket fire it or if you have a problem with a torn wrap or a dented battery contact you need to get it out of your pocket as quickly as possible. If you have pocket fired you stand a fair chance of managing it - in the other two instances you will probably spray your groin with white hot chemicals.
All atomisers must have a fixed 510 - forgetting this and fitting a stock coil tank that has a floating 510 can cause a hard short - another recipe for batteries venting and spraying white hot chemicals .
The power will never remain constant as it will reduce in line with the battery voltage.
You will probably need to change the battery twice as frequently as you would in a fully regulated mod as the vape from a mech can be underpowered and insipid in a mech once the battery is below half charge.
You have to clean them constantly to ensure they work properly. Even if you clean it after use and put it on your vape shelf you will probably need to do it again the next time you use it as copper and brass tarnishes really fast.
I like mechs and have a shelf that is just mech mods - but I'd estimate I use fully regulated mods to vape with over 90% of the time.
If you're in any doubt mechs are best left alone. Although I like them I wouldn't recommend them to anyone.