1) Round-up in concentrate is very cost effective. Especially if applied with a hand sprayer. I've had 1 bottle of concentrate for about 8 years and have a big yard and a long driveway that wants to grow weeds in the cracks
2) I don't have any 3 headed (you name the critter)frogs or toads around here. Been using for 25 years at this house.
3) Glycosphates (brand name Round-Up) work best on grasses but works well on some broadleaf weeds.
Round-Up I was under the impression is 1) Expensive 2) Not environmentally friendly, and 3) Works on weeds but not on grass and stuff (which I need to kill too in the concrete.)
1) depends on what you consider expensive 2) since vegetation is part of the 'environment' and Round-Up kills all vegetation, one could say it's bad for the "environment". 3) it is a non-selective herbicide... kills all plants**
Given our daily chances of rain this time of year, you'd be better off applying one of the consumer products that 'works' quickly vs the slower commercial products.
**some 'woody' plants are somewhat resistant, but I doubt you have many of those growing in your driveway cracks.