Boarding an enemy spacecraft with the intent of capturing it is extremely difficult, especially if the vessels are still in fighting condition. Raids (ie. smaller attacks meant to damage - or possibly destroy - an enemy starship) are somewhat easier to accomplish, but unless your troops has a way to retreat it's a death sentence for all those involved (not necessarily an issue for certain factions).

There are several reasons why boarding is difficult. For one it is difficult to get boarders across to an enemy vessel still under power; the ship moves, it shoots back, it has void shields and thick armour, all of which complicates the matter. Provided you can get your men aboard it's a matter of size; starship as so vast. Even an escort-sized ship is several kilometres long and masses millions upon millions of tons, with endless warrens of corridors and tens of thousands of crew. And you can bet they know their ship better than you do. They'll block your advance with heavy doors, vent the atmosphere, turn of gravity - and all the while they will be coming at your from multiple directions and/or forcing you into murderous strong-points. And they won't be armed with clubs and shotguns either; when a ship is in danger of being take a prize the gloves come off and the armouries hand out the good stuff, collateral damage be damned. Even if you get hundreds - or even thousands - of warriors on board it might not be enough against a determined defender. Capital ships are even worse of course.

Only Space Marines using boarding torpedoes and/or Thunderhawk gunships have any real hope of capturing enemy combat vessels relatively intact or routinely carrying out hit-and-run raids. Only space marines have the specialized equipment, the level of training, and the special skills required. The thing with space marines is that they have the ability to get their warriors onto your ship. And they have the extreme firepower and mobility that allow them to keep advancing no matter what. Which is essential for any boarder - if you get bogged down you're going to die. The best Astartes assault troops are able to turn the ship against itself so to speak - they move so quickly and so violently that the defending crew cannot compensate quickly enough and become trapped and isolated, unable to mount a coordinated defence.

Other factions with a penchant for boarding actions include orks (they enjoy great numbers and generally doesn't mind severe casualties) and non-Astartes Chaos (much the same as the orks). Eldar reavers are also fond of raids, striking and then fading before the enemy can react, but generally avoid major boarding actions because of their limited numbers and light equipment.

Using Astartes an example: As a rule of thumb you need a full company of 100 to have any chance of seizing an enemy escort (half that to conduct a meaningful raid). Success is no guaranteed and there is a good chance your losses will be significant. To capture an enemy cruiser you must either have an overwhelming number of Astartes (at the very least three companies, preferably more) or deploy a company of Terminators.

A viable boarding operation usually looks something like this:

Softening up the target - Unless you're feeling suicidal it's usually a good idea to soften up the target. Each hull hit will mean more ways into the enemy vessel, fewer defenders to fight, and generally more chaos and confusion (which is good for the boarders). Unfortunately more damage means the ship is worth less as a prize - capturing a floating hulk is pretty useless. So the trick is to find some sort of balance here.

Getting into range - To board using torpedoes or teleporters you have to get into short range. That means your ships have to run the gauntlet of fire first (which is why Strike Cruisers and Battle Barges are so good at it). Teleporters are by far the best means of boarding, but they are rare, have limited capacity, and require the target's void shields to be down. Torpedoes have no such limitation, but can be shot down by defensive turrets (albeit with great difficulty). Assault boats (including Thunderhawks) have longer ranges, but have the added complexity of needing to avoid getting shot down before they can board. Teleporters and assault boats are also great for getting raiders out again, boarding torpedoes much less so.

Establishing beachheads - The more men you have the more ways in you need; you don't want to create a bottleneck where your own numbers are your biggest obstacle. Boarding torpedoes can usually make their own ways in except in the face of very heavy armour plating (which is good since they aren't all that accurate). Assault boats are more complicated. If the targets has suffered significant hull damage there may be some routes in, but otherwise the men have to secure some docking points. Teleporters are great for establishing beachheads, but keep in mind they aren't all that accurate. Chances are there will be some scatter - you might aim for the bridge, but your Terminator squads could end up all over the ship's superstructure, greatly complicating matters.

Reaching objectives: To successfully capture an enemy vessel you have to take control over the command deck (aka. bridge) and the enginarium. That's the absolute minimum. From whatever beachheads you've established your boarders will work their way towards their objectives at best speed from multiple (one hopes) directions. It's all about speed and violence now. The layout of the ships greatly favour the defender, so you cannot lose the initiative. If you stop moving, you fail to reach your objectives - and then you die. A more thorough capture action involves securing main gun decks, launch decks, armouries, arterial access-ways, key utility facilities and subsystems, communications and navigation, secondary and auxiliary controls, and much more.

To sum it up. Astartes are the only faction to routinely rely on boarding as part of fleet operations, and even they must expect heavy casualties from time to time. Orks and Chaos board whenever they get the chance. In the case of the orks that's most versus severely damages ships or when they have the advantage of overwhelming numbers. Chaos fleets will use boarding as yet another weapon at their disposal, but not one they rely on to carry the day. Eldar reavers raid to cause damage. Other factions are largely content to avoid getting boarded.