I assume you're referring to the Vinland Saga?

Askeladd's design with the cuirass isn't accurate no and probably designed just to make his character more distinctive in appearance. As the commander of a small band of Danes he might posses a mail shirt on the body this late into the viking age, short sleeved and short on the body most likely. This would likely be accompanied, at least in battle, by a nasalised or spectacled helm.

Gratainus' design, like his ships, is more or less a dramatic flair rather than being particularly historically accurate, attempting to strengthen the character's legendary connection to Lucius Artorius Castus in the show. While it is true some Welsh kingdoms liked to trace their histories back to Romano-Briton leaders of the 5th century like Vortigern. With Wales and Cornwall being the last major refugees of independent Briton kingdoms after saxon invasions but those kingdoms were far more Briton than Roman in nature, and this extends to dress sense. The claims made about Artorius are also highly inaccurate and far more closely resemble men like Vortigern who probably was a real person or Ambrosius Aurelianus but i wouldn't trust much about what Gildas or Nennius have to say about the latter in particular.

The cloak is perhaps accurate, a good welsh, particularly in the cochli style, cloak was worn as a symbol of some rank with welsh battledress much more than say their English counterparts although a brighter colour might be more appropriate. But their body armour would be much more likely to be a mail shirt if he wears some, probably with relatively short sieves and shorter on the body than later kinds of mail. There's a few possibilities for the helmet but the most likely for a welshman of his rank would be a peaked helm, probably with a nasal piece and in the celtic style sometimes cheek pieces. I believe we're still a little early for a Chausses, mail that cover the legs, at least among welshmen who'd be a bit behind in that kind of armour tech compared to people like the French.

The welsh soldiers shown in the next episode are reasonable, mostly unarmoured or lightly armoured spearmen with long Gwaywffon spears designed to be used in one or both hands and round wooden shields, though most depictions of them I've seen paint their shields and the spear's shown are probably a little short although the spearhead is slightly exaggerated in how diamondy the shape is but not a bad match. Conversely where most depictions of soldiers of this kind of period make their clothing way too uniform welsh warriors, at least those Rhyfelwr warriors who were a little more professional, were mostly likely to have a slightly greater uniformity in the colour of their clothing. Particularly if these are slightly more professional soldiers they'd also be likely to be carrying javelins and long knives in addition to the Gwyawaffon. In addition to this the welsh archers aren't an unreasonable depiction and although the welsh hadn't begun to use their famous large, welsh long bows as real weapons of war yet archery was still valued among the welsh kingdoms with the kinds of skrimish tactics loosely depicted in the stand off in episode 13 being not a terribly inaccurate one.