Effects on gardens I can't say much on. But I can tell you about one extremely important aspect of frogs that MUST be considered in suburbia. If you want to know what this issue is without reading my context setting and almost :-) relevant preamble, skip to the heading "Cut to the chase ..." below.
I live in suburbia in a 1/4 acre jungle "just off" Auckland City. (ie people think that I live at a residential address in a residential street but I really live in a semi jungle :-) ).
We have what was an above ground concrete swimming pool about 35 feet x 15 feet, but for many years it's been a home for such wild ducks that wish to visit, or top stay and raise families. We probably get 5 to 15 new families of (wild) ducklings per year. Mallards seem to have claimed the property as their own. We get very rarely a Grey Heron (sorry guy, fish are all gone) and a steady trickle of NZ native birds (Tui, Piwakawaka, occasional Kereru.) But, I (almost) digress :-).
And, we have two small goldfish and "lily" ponds. The lilies and goldfish are long gone, victims of the ducks. But we get frogs. Big noisy ones. I don't know how they got here or how they found us. But they seem to like what they have found. During the winter months there is no sign of them. Where they go to and/or how they hide I know not. But, sometime in the spring as the days lengthen and the nights warm, suddenly they are there.
Cut to the chase ...
How do I know? You can't miss them.
"Braaaaaaaach. Braaaaaach. Braaaaaaaach.
... pause ...
wough wough wough wough wough.
... Answer in kind by another frog, or if not, a longer pause.
Ad all night longeum.
Very very loud.
They do not bother me. I like the sound.
I can sleep through almost anything and can roll over and go back to sleep in 5 minutes if woken. My wife can't. Our bedroom is at the front of a largish house. The frog pond is at the rear and then some 30 feet further back again. For the last two years at least they have moved slightly further away again, into the "duck pond" / swimming pool. STILL VERY loud. Still wife waking. They are closer to the neighbours. Pretty grim on occasion, I'm told. Odds are that they may have to go. Quite how you catch large fairly invisible deep pool enshrined frogs I know not. But, I may yet have to find out.
If I had a vegetable garden instead of a jungle I'd expect that the frogs would be of at least minor benefit and unlikely to be harmful. The jungle does not seem to mind at all.