I've heard that Mars is tough because the gravity is high enough that you need a lot of braking to land, but it's not quite high enough to maintain a thick atmosphere, so you can't use drag (e.g parachutes) to help reduce work/weight. Venus has a thick atmosphere and the Venera probes were able to use parachutes to help land.

There've been so few successful Mars landings, that this could be interpreted as "the Soviets lost a couple of dice throws", really; it's not clear how much of a success is down to doing it properly vs just luck. Everyone has failures with Mars.

Also, of course, they clearly put more resources into Venus. AIUI it was the first Venera landings that established once and for all just how harsh conditions on Venus are; before that it looked potentially less harsh, and more interesting than, Mars.

Mars is expensive to get to and getting there is the hard part whereas Venus is cheap to get to and existing once you're there is the hard part. This has big implications for probe design.

A successful Mars probe is very light for the amount of science bits it packs (mars is far away so each probe is expensive, gotta pack as much science as possible on each one). As long as your stuff works after touchdown you're basically in the clear since the environment is pretty tolerable and generally stable. It's an exercise in value engineering everything to within a hair of its life so that you can fit it within the weight budget and get it to Mars in the first place. This sort of compromise game is something American engineering and project management tends to excel in.

Venus on the other hand is an exercise in pruning down your science instrument wish list to just the bare essentials (Venus is decently close, no need to compromise things so your favorite instrument can make the cut, it can just catch a ride on the next probe) then packaging all that in a form factor that can operate and survive as long as possible as the nearly four digit outside temperatures slowly overwhelm the probe. It's an exercise in picking the bare essentials that you need to do the job and then building it ruggedly enough to do that job consistently in adverse conditions and even as conditions degrade beyond what the system is designed to work in is something Russian engineering and project management tends to excel in.

One reason I read about once was that with Soviet rockets it was much more easy and economically feasible to get to Venus than to Mars, due to less delta-V required for inner Solar system travel. Being able to successfully land is irrelevant if you never get into planet’s orbit in the first place.

True. Mars is a "cursed" planet though. European lander has failed there also. NASA really knocked it out of the park with the rovers. They're the only ones who know how to really do it properly.