Back in the day, my first Devastator squad was armed with one lascannon and three plasma cannons. It didn't cause any issues on the field, because I rarely played them. So, I never expanded them beyond the requisite sergeant and the gunners. Back in the 3rd edition days, the Devastator squads had to pay double for their heavy weapons. That is the main reason why they almost never saw the field among Space Marine players.
I slowly expanded the one squad into two 5 man squads, one with lascannons, and the other with just plasma. Even then, they were still underarmed as I never added the fourth plasma cannon. It wasn't until much later that I added an extra 5 guys to each squad to make them full 10 man squads.
Half of the two squads are my wave one guys. The rest are all wave two marines, but backed down to fit in with the rest of the unit. If you look at the plasma cannons, you can probably figure out which one is the newest. You may also figure out which las cannon gunner is the original.
Anyway, in play, I rarely field them without combat squadding them. The plasma cannons do great when operating in two units of two cannons in each. Four in a single squad really is overkill. With them split up, they really can cover each other pretty well and add a real deterrent to deep striking units or denying objectives.
The lascannon squad can go multiple ways. Leaving them as a single 10 man squad leaves quite a few ablative wounds to make the four lascannons even more durable and dangerous to Riptides, Land Raiders, etc. Splitting them up again makes for two squads of practically twin-linked lascannons for when you absolutely want to hit them hard. Even squadding one into just 5 standard marines can allow for them to advance and capture or even harass the enemy, while the four lascannons wreak havoc.