This means on your opponent’s turn you’ll get a +1 to hit in melee, or with any out of sequence stuff that may occur (except when Overwatching of course), which is a fantastic offensive boost, as well as the inherent advantages of getting a +1 in your turn as well. Ignoring Combat Attrition outright is useful to have, but the kicker is the +1 to hit so long as you’ve not moved this turn, pile ins and consolidations being the exception.
Most units can hold their own without support but many strategies rely on Characters as a lynchpin, so losing them due to poor positioning or certain abilities can make the army feel a lot less effective.Using those supplements and rules means books, a single Space Marine army will use at minimum of two books- Codex and Chapter Supplement, with more depending on your army (if you have multiple Chapters or other Allies), and who knows what else that might be released.And as they kill Marines well, they’ll kill other units well, meaning they’ll be a common occurrence.
Even with their good statline there are many, many methods and weapons which are designed to kill Marines.Despite a large model range and options, some are just vastly better than others on paper and in practice, leading to a lot of similar looking armies competitively.