What are the effects of stopping concerta without weaning off ?

None that are seriou: When one stops concerta, one no longer gets the therapeutic benefit. If it was taken as prescribed, there should be no withdrawal syndrome.

Nothing: You essentially wean off it by the end of each day anyway. It is gone from your system unless you take it every 8-12 hours. Your emotional dependence on using it as a crutch may be impacted, but there is no wean needed like many of the meds for other issues.